Plated steel sheet with quasicrystal

ABSTRACT

A plated steel sheet with a quasicrystal includes a steel sheet and a plated-metal-layer arranged on a surface of the steel sheet. The plated-metal-layer includes, as a chemical composition, Mg, Zn. The plated-metal-layer includes, as a metallographic structure, a quasicrystal phase. A Mg content, a Zn content, and an Al content in the quasicrystal phase satisfy 0.5≤Mg/(Zn+Al)≤0.83 in atomic %. In addition, an average equivalent circle diameter of the quasicrystal phase is equal to or larger than 0.01 μm and equal to or smaller than 1 μm.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a surface-treated steel sheet which isexcellent in corrosion resistance. Particularly, the present inventionrelates to a plated steel sheet with a quasicrystal.

RELATED ART

The quasicrystal is the crystal structure which was firstly discoveredin 1982 by Dr. Daniel Shechtman, and has an atomic arrangement with apolyhedron with 20 faces (icosahedron). The quasicrystal is known as thecrystal structure which has unique rotational symmetry not to beobtained by general metals and alloys, is a non-periodic crystalstructure having fivefold symmetry for example, and is equivalent to anon-periodic structure represented by a three-dimensional PenrosePattern.

After the discovery of the new arrangement of metallic atoms, that isnew crystal structure, the quasicrystal having the quasi-periodicstructure and having the unique rotational symmetry has received a lotof attention. The quasicrystal has been generally obtained by a liquidquenching method in the past, although it is found that the quasicrystalcan be obtained by a crystal growth method in recent years. The shapethereof has been restricted to powder, foil and chip, and thus, it hasbeen very rare to apply the quasicrystal to a product.

Patent documents 1 and 2 disclose high strength Mg based alloys andproducing methods thereof. The Mg based alloys have a metallographicstructure in which the hard quasicrystal phase having a grain size oftens to hundreds of nm is dispersedly precipitated, and thus, the Mgbased alloys are excellent in strength and elongation. The patentdocuments 1 and 2 utilize the properties such that the quasicrystal ishard.

Patent document 3 discloses a thermoelectric material using Al basedquasicrystal. The patent document 3 utilizes the properties such thatthe quasicrystal is excellent in thermoelectric property. Patentdocument 4 discloses a heat-resistant catalyst whose precursor is aquasicrystalline Al alloy (Al based quasicrystal) and a producing methodthereof. The patent document 4 utilizes the properties such that thequasicrystal without a periodic crystal structure is brittle andfracturable. As described above, in the prior inventions, thequasicrystal is often dispersed as fine particles or the quasicrystalbeing fine particles is often consolidated.

As another application different from the above inventions, patentdocument 8 discloses a metallic coating for cookware containing thequasicrystal. In the patent document 8, the coating excellent in wearresistance and corrosion resistance to salt is applied to the cookwareby plasma-spraying the alloy powder containing the quasicrystal whichconsists of Al, Fe, and Cr and which is excellent in corrosionresistance.

As described above, the Mg based quasicrystal is utilized as thematerials excellent in strength, and the Al based quasicrystal isutilized as members which is excellent in strength, thermoelectricmaterials, and coatings for cookwares, or the like. However, theseutilizations are limited, and the quasicrystal is not always utilized inmany fields.

The quasicrystal has excellent characteristics derived from the uniquecrystal structure. However, the characteristics thereof are onlypartially investigated, and the quasicrystal is not widely applied toindustrial fields at the moment. The present inventors have tried toimprove the corrosion resistance by applying the quasicrystal which ishardly utilized in the industrial field to a plated-metal-layer of asurface-treated steel sheet.

In general, in order to prolong a useful life of steel sheet, the steelsheet is subjected to surface treatment such as metallic plating, paintcoating, conversion coating, or organic film laminating in order toensure an anticorrosive function to a certain extent. In the many steelmaterials used in fields of automobiles, consumer electronics, buildingmaterials or the like, the metallic plating is mainly applied. Theplated-metal-layer provides, at a low cost, both barrier protection inwhich a base metal (steel substrate) is shielded from outsideenvironment and sacrificial protection in which the layer ispreferentially corroded as compared with the base metal.

There are various methods to industrially form the plated-metal-layer.In order to make the plated-metal-layer thick, spraying, hot-dip platingor the like is preferable. In order to uniformly form theplated-metal-layer, sputtering, ion plating, evaporating, electroplating or the like is preferable. Among these methods, the hot-dipplating is widely applied, because it is possible to massively andeconomically produce the steel materials with the plated-metal-layer.

In the electro plating, deposited metals are limited, and thus, theelements included in the plated-metal-layer are limited in general. Inthe methods such as the spraying and the evaporating in which theplated-metal-layer is formed by using reactions such as melting,evaporation, deposition, and solidification of metals, it is possible toform the plated-metal-layer as with that formed by the hot-dip platingin theory. However, each metal has specific melting point and boilingpoint, and thus, the difference between chemical compositions of theused alloy and the formed plated-metal-layer tends to occur in thespraying and the evaporating.

Since it is possible for the hot-dip plating to form theplated-metal-layer whose chemical composition is about the same as thatof the used alloy for the hot-dip bath, the hot-dip plating is wellsuitable for forming the plated-metal-layer which has predeterminedchemical composition as compared with other forming methods.

At present, conventional surface-treated steel sheets which areindustrially available are mainly those with the plated-metal-layer ofZn-based alloy or Al-based alloy. The plated-metal-layer of Zn-basedalloy includes Zn as main element and a small amount of Al, Mg, or thelike, and the metallographic structure thereof includes Zn phase, Alphase, Mg₂Zn phase, or the like. The plated-metal-layer of Al-basedalloy includes Al as main element and a small amount of Si, Fe, or thelike, and the metallographic structure thereof includes Al phase, Siphase, Fe₂Al₅ phase, or the like.

As the plated steels in which the chemical composition of plated alloyis quite different from that of the conventional surface-treated steelsheets, the present inventors disclosed the steel sheets with the platedlayer containing Mg-based alloy in patent documents 5 to 7. Based on theabove plated steels, the present inventors have tried to further improvethe corrosion resistance by focusing the quasicrystal which has hardlybeen considered for the improvement of the corrosion resistance of theplated layer (plated-metal-layer).

The present inventors considered a metallographic structure whichpreferably improve the corrosion resistance particularly by focusing ondispersing quasicrystals having an average equivalent circle diameter ofequal to or smaller than 1 μm in the plated-metal-layer. As describedabove, the quasicrystal is known as the crystal structure which hasunique rotational symmetry not to be obtained by general metals andalloys, is a non-periodic crystal structure having fivefold symmetry forexample, and is equivalent to a non-periodic structure represented by athree-dimensional Penrose Pattern.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2005-113235

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2008-69438

[Patent Document 3] Japanese Unexamined Patent Application, FirstPublication No. H-08-176762

[Patent Document 4] Japanese Unexamined Patent Application, FirstPublication No. 2004-267878

[Patent Document 5] Japanese Unexamined Patent Application, FirstPublication No. 2008-255464

[Patent Document 6] Japanese Unexamined Patent Application, FirstPublication No. 2010-248541

[Patent Document 7] Japanese Unexamined Patent Application, FirstPublication No. 2011-219823

[Patent Document 8] Published Japanese Translation No. 2007-525596 ofthe PCT International Publication

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a plated steel sheetwhich is rapidly improved in the corrosion resistance requested forapplying building materials, automobiles, consumer electronics or thelike.

In particular, an object of the present invention is to provide theplated steel sheet having both excellent corrosion resistance andexcellent sacrificial protection by focusing the quasicrystal which hashardly been considered for the improvement of the corrosion resistanceof the plated layer and by clarifying the morphology of themetallographic structure which maximally improve the corrosionresistance. Specifically, the corrosion resistance and the sacrificialprotection of the plated steel sheet is to be improved by clarifying thepreferable morphology of the quasicrystal in plated-metal-layer (theplated layer) which has hardly been considered but which is expected toimprove the corrosion resistance and by clarifying the processes topreferably form the quasicrystal in the plated-metal-layer.

Means for Solving the Problem

An aspect of the present invention employs the following.

(1) A plated steel sheet with a quasicrystal according to an aspect ofthe present invention includes a steel sheet and a plated-metal-layerarranged on a surface of the steel sheet,

wherein: the plated-metal-layer includes, as a chemical composition, byatomic %,

28.5% to 52% of Zn,

0.5% to 10% of Al,

0% to 3.5% of Ca,

0% to 3.5% of Y,

0% to 3.5% of La,

0% to 3.5% of Ce,

0% to 0.5% of Si,

0% to 0.5% of Ti,

0% to 0.5% of Cr,

0% to 2% of Fe,

0% to 0.5% of Co,

0% to 0.5% of Ni,

0% to 0.5% of V,

0% to 0.5% of Nb,

0% to 0.5% of Cu,

0% to 0.5% of Sn,

0% to 0.2% of Mn,

0% to 0.5% of Sr,

0% to 0.5% of Sb,

0% to 0.5% of Pb, and

a balance of Mg and impurities;

the plated-metal-layer includes, as a metallographic structure, aquasicrystal phase;

a magnesium content, a zinc content, and an aluminum content expressedin atomic % in the quasicrystal phase satisfy 0.5≤Mg/(Zn+Al)≤0.83; and

an average equivalent circle diameter of the quasicrystal phase is equalto or larger than 0.01 μm and equal to or smaller than 1 μm.

(2) In the plated steel sheet with the quasicrystal according to (1),

a calcium content, an yttrium content, a lanthanum content, and a ceriumcontent expressed in atomic % in the chemical composition of theplated-metal-layer may satisfy0.3%≤Ca+Y+La+Ce≤3.5%.

(3) In the plated steel sheet with the quasicrystal according to (1) or(2),

a silicon content, a titanium content, and a chromium content expressedin atomic % in the chemical composition of the plated-metal-layer maysatisfy0.005%≤Si+Ti+Cr≤0.5%.

(4) In the plated steel sheet with the quasicrystal according to any oneof (1) to (3),

a zinc content and an aluminum content expressed in atomic % in thechemical composition of the plated-metal-layer may satisfy30%≤Zn+Al≤52%.

(5) In the plated steel sheet with the quasicrystal according to any oneof (1) to (4),

when viewed in a cross section, whose cutting direction is parallel to athickness direction of the plated-metal-layer,

the metallographic structure of the plated-metal-layer may be a bimodalstructure which comprises a fine domain composed of a grain having anequivalent circle diameter of 0.2 μm or smaller and a coarse domaincomposed of a grain having an equivalent circle diameter of larger than0.2 μm,

the coarse domain may include at least one selected from thequasicrystal phase, a Zn phase, an Al phase and a MgZn phase,

the fine domain may include at least one selected from a Mg₅₁Zn₂₀ phase,a Zn phase, an amorphous phase, a Mg₃₂(Zn,Al)₄₉ phase, and

the average equivalent circle diameter of the quasicrystal phase may belarger than 0.2 μm and equal to or smaller than 1 μm.

(6) In the plated steel sheet with the quasicrystal according to any oneof (1) to (4),

when viewed in a cross section, whose cutting direction is parallel to athickness direction of the plated-metal-layer,

the metallographic structure of the plated-metal-layer may be a bimodalstructure which comprises a fine domain composed of a grain having anequivalent circle diameter of 0.2 μm or smaller and a coarse domaincomposed of a grain having an equivalent circle diameter of larger than0.2 μm,

the coarse domain may include at least one selected from a Zn phase, anAl phase and a MgZn phase,

the fine domain may include at least one selected from the quasicrystalphase, a Mg₅₁Zn₂₀ phase, a Zn phase, an amorphous phase, a Mg₃₂(Zn,Al)₄₉phase, and

the average equivalent circle diameter of the quasicrystal phase isequal to or larger than 0.01 μm and equal to or smaller than 0.2 μm.

(7) In the plated steel sheet with the quasicrystal according to any oneof (1) to (5),

an area fraction of the coarse domain in the metallographic structuremay be equal to or more than 5% and equal to or less than 50%, and

an area fraction of the fine domain in the metallographic structure maybe equal to or more than 50% and equal to or less than 95%.

(8) In the plated steel sheet with the quasicrystal according to any oneof (1) to (4), and (6),

an area fraction of the coarse domain in the metallographic structuremay be equal to or more than 5% and equal to or less than 50%, and

an area fraction of the fine domain in the metallographic structure maybe equal to or more than 50% and equal to or less than 95%.

(9) In the plated steel sheet with the quasicrystal according to any oneof (1) to (5), and (7),

an area fraction of the quasicrystal phase included in the coarse domainmay be equal to or more than 80% and less than 100% in the coarsedomain, and

an area fraction in total of the Mg₅₁Zn₂₀ phase, the Zn phase, theamorphous phase, and the Mg₃₂(Zn,Al)₄₉ phase included in the fine domainmay be equal to or more than 80% and less than 100% in the fine domain.

(10) In the plated steel sheet with the quasicrystal according to anyone of (1) to (4), (6), and (8),

an area fraction in total of the Zn phase, the Al phase, and the MgZnphase included in the coarse domain may be equal to or more than 80% andless than 100% in the coarse domain, and

an area fraction of the quasicrystal phase included in the fine domainmay be more than 0% and less than 10% in the fine domain.

(11) In the plated steel sheet with the quasicrystal according to anyone of (1) to (5), (7), and (9),

when viewed in the cross section and when a thickness of theplated-metal-layer is regarded as D, an area from a surface of theplated-metal-layer toward the steel sheet in the thickness direction to0.05×D is regarded as an outermost area of the plated-metal-layer, andan area from an interface between the steel sheet and theplated-metal-layer toward the plated-metal-layer in the thicknessdirection to 0.05×D is regarded as an innermost area of theplated-metal-layer,

an area fraction of the coarse domain in the outermost area of theplated-metal-layer may be equal to or more than 7% and less than 100%and an area fraction of the coarse domain in the innermost area of theplated-metal-layer may be equal to or more than 7% and less than 100%,and

when an area except for the outermost area and the innermost area of theplated-metal-layer is regarded as a main-body area of theplated-metal-layer,

an area fraction of the fine domain in the main-body area of theplated-metal-layer may be equal to or more than 50% and less than 100%.

(12) In the plated steel sheet with the quasicrystal according to anyone of (1) to (4), (6), (8) and (10),

when viewed in the cross section and when a thickness of theplated-metal-layer is regarded as D, an area from a surface of theplated-metal-layer toward the steel sheet in the thickness direction to0.05×D is regarded as an outermost area of the plated-metal-layer, andan area from an interface between the steel sheet and theplated-metal-layer toward the plated-metal-layer in the thicknessdirection to 0.05×D is regarded as an innermost area of theplated-metal-layer,

an area fraction of the coarse domain in the outermost area of theplated-metal-layer may be equal to or more than 7% and less than 100%and an area fraction of the coarse domain in the innermost area of theplated-metal-layer may be equal to or more than 7% and less than 100%,and

when an area except for the outermost area and the innermost area of theplated-metal-layer is regarded as a main-body area of theplated-metal-layer,

an area fraction of the fine domain in the main-body area of theplated-metal-layer may be equal to or more than 50% and less than 100%.

(13) In the plated steel sheet with the quasicrystal according to anyone of (1) to (12),

a Mg phase may be absent in the metallographic structure of theplated-metal-layer.

(14) The plated steel sheet with the quasicrystal according to any oneof (1) to (13) may further include a Fe—Al containing alloy layer,

the Fe—Al containing alloy layer may be arranged between the steel sheetand the plated-metal-layer,

the Fe—Al containing alloy layer may include at least one selected fromFe₅Al₂ and Al_(3.2)Fe, and

a thickness of the Fe—Al containing alloy layer may be equal to or morethan 10 nm and equal to or less than 1000 nm.

(15) A method of producing a plated steel sheet with a quasicrystalaccording to an aspect of the present invention, which is the method ofproducing the plated steel sheet with the quasicrystal according to anyone of (1) to (14), includes:

a hot-dip-plating process of dipping a steel sheet into ahot-dip-plating bath having an adjusted composition in order to form aplated-metal-layer on a surface of the steel sheet;

a first cooling process of cooling the steel sheet after thehot-dip-plating process under conditions such that an average coolingrate of the plated-metal-layer is equal to or faster than 15° C./sec andequal to or slower than 50° C./sec in a temperature range where atemperature of the plated-metal-layer is from T_(melt)+10° C. toT_(solid-liquid), when the T_(melt) in unit of ° C. is regarded as aliquidus temperature of the plated-metal-layer and when theT_(solid-liquid) in unit of ° C. is a temperature range where theplated-metal-layer is in a coexistence state of a solid phase and aliquid phase and where a volume ratio of the solid phase to theplated-metal-layer is equal to or more than 0.01 and equal to or lessthan 0.1; and

a second cooling process of cooling the steel sheet after the firstcooling process under conditions such that an average cooling rate ofthe plated-metal-layer is equal to or faster than 100° C./sec and equalto or slower than 3000° C./sec in a temperature range where atemperature of the plated-metal-layer is from a temperature at finishingthe first cooling process to 250° C.

(16) In the method of producing the plated steel sheet with thequasicrystal according to (15), in the hot-dip-plating process:

an oxygen concentration of an atmosphere at dipping the steel sheet maybe 100 ppm or less in volume ratio;

a plating tub to hold the hot-dip-plating bath may be a steel tub;

T_(bath) which is a temperature of the hot-dip-plating bath may be equalto or higher than 10° C. and equal to or lower than 100° C. higher thanthe T_(melt); and

a time for dipping the steel sheet into the hot-dip-plating bath may beequal to or longer than 1 sec and equal to or shorter than 10 sec.

Effects of the Invention

According to the above aspects of the present invention, it is possibleto provide the plated steel sheet which is further excellent in thecorrosion resistance requested for applying building materials,automobiles, consumer electronics or the like. Therefore, it is possibleto prolong the useful life of the materials as compared with theconventional surface-treated steel sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM micrograph of a plated steel sheet according to anembodiment of the present invention and a metallographic micrographobtained by observing a cross section whose cutting direction isparallel to a thickness direction of the plated steel sheet.

FIG. 2 is a TEM micrograph of a plated steel sheet according to theembodiment and a metallographic micrograph obtained by observing thecross section whose cutting direction is parallel to the thicknessdirection of the plated steel sheet.

FIG. 3 is a TEM micrograph of a plated steel sheet according to theembodiment and a metallographic micrograph obtained by observing thecross section whose cutting direction is parallel to the thicknessdirection of the plated steel sheet.

FIG. 4A is an electron diffraction pattern obtained from a local area 2a 1 in a coarse domain 2 a shown in FIG. 2.

FIG. 4B is an electron diffraction pattern obtained from a local area 2b 1 in a fine domain 2 b shown in FIG. 2.

FIG. 5A is an electron diffraction pattern obtained from a local area 2a 2 in a coarse domain 2 a shown in FIG. 3.

FIG. 5B is an electron diffraction pattern obtained from a local area 2b 2 in a fine domain 2 b shown in FIG. 3.

FIG. 6 is an equilibrium state diagram of a binary Zn—Mg system.

FIG. 7 is a liquidus surface phase diagram of a ternary Zn—Al—Mg system.

EMBODIMENTS OF THE INVENTION

Hereinafter, a preferable embodiment of the present invention will bedescribed in detail. However, the present invention is not limited onlyto the configuration which is disclosed in the embodiment, and variousmodifications are possible without departing from the aspect of thepresent invention.

The plated steel sheet according to the embodiment includes a steelsheet (base metal) and a plated-metal-layer (plated layer) arranged on asurface of the steel sheet. The plated-metal-layer whose shape is thinfilm is the alloy which has adhesion to the base metal, equips the steelsheet with a function such as anticorrosion, and does not harm theproperties of the base metal such as strength or rigidity. Specifically,the plated steel sheet according to the embodiment is the compositematerial in which two types of metal-alloy-materials that are the steelsheet and the plated-metal-layer are layered. In an interface betweenthe steel sheet and the plated-metal-layer, an interface alloy layer(Fe—Al containing alloy layer) or a diffused area formed by mutualdiffusion of metal atoms may exist as a result of the composition, andthereby, the interface adherence may be increased due to atomic bondingof metal. First, the properties requested to the plated-metal-layer ofthe plated steel sheet according to the embodiment will be described.

The plated-metal-layer of the plated steel sheet is required to beexcellent in anticorrosion performance. The anticorrosion performance isclassified into the corrosion resistance and the sacrificial protection.In general, the corrosion resistance of the plated-metal-layercorresponds to the corrosive resistivity of the plated-metal-layeritself, and is usually evaluated by the corrosion loss of theplated-metal-layer after a predetermined time in various corrosiontests.

When the corrosion loss is small, the plated-metal-layer remains for along time as a protective coat of the steel sheet (base metal), andthus, the corrosion resistance is excellent. When the corrosion loss isevaluated by using pure metals, the corrosion resistance of Zn tends tobe better than that of Mg, and the corrosion resistance of Al tends tobe better than that of Zn in general.

On the other hand, the sacrificial protection of the plated-metal-layercorresponds to the protective function for the steel sheet in which theplated-metal-layer is preferentially corroded instead of the steel sheetwhen the steel sheet is accidentally exposed in corrosive environment.When the sacrificial protection is evaluated by using pure metals, themetal which is electrochemically less-noble and which tends to becorroded is excellent in the sacrificial protection. Thus, thesacrificial protection of Zn tends to be better than that of Al, and thesacrificial protection of Mg tends to be better than that of Zn ingeneral.

The steel sheet plated the Zn—Mg alloy according to the embodimentincludes a large amount of Mg in the plated-metal-layer, and thus, isexcellent in the sacrificial protection. On the other hand, the point tobe improved is to reduce the corrosion loss of the plated-metal-layer,which is to improve the corrosion resistance of the plated-metal-layer.

The present inventors have investigated a constituent phase of themetallographic structure of the plated-metal-layer in order topreferably reduce the corrosion loss of the plated-metal-layer in thesteel sheet plated the Zn—Mg alloy. As a result, it is found that thecorrosion resistance is drastically improved by including thequasicrystal phase in the plated-metal-layer.

The metallographic structure of the plated-metal-layer is the maincharacteristic of the plated steel sheet according to the embodiment. Ina case where the plated steel sheet is produced based on a chemicalcomposition within a specific range, which will be described later,under specific production conditions, a quasicrystal phase is formed inthe plated-metal-layer, and corrosion resistance can be significantlyimproved. In the embodiment, an average equivalent circle diameter(diameter) of the quasicrystal phase formed in the plated-metal-layer isequal to or larger than 0.01 μm and equal to or smaller than 1 μm.

The plated-metal-layer of the plated steel sheet according to theembodiment contains the aforementioned quasicrystal phase. Therefore,the corrosion resistance thereof is further improved compared to thecorrosion resistance of a plated-metal-layer not containing aquasicrystal phase. Furthermore, the plated-metal-layer of the platedsteel sheet according to the embodiment contains a large amount of Mg.Therefore, the plated-metal-layer also exhibits excellent sacrificialprotection with respect to the steel sheet. That is, the plated steelsheet according to the embodiment includes an ideal plated-metal-layerexcellent in both of the corrosion resistance and the sacrificialprotection.

Hereinafter, regarding the plated steel sheet according to theembodiment, the chemical composition of the plated-metal-layer, themetallographic structure of the plated-metal-layer, and the productionconditions will be specifically described in this order.

Generally, when constitutive equations of metallic phases orintermetallic compounds such as Zn, Al, Mg₂Zn, and Fe₂Al₅ are described,an atomic ratio is used instead of a mass ratio. The embodiment will bedescribed using an atomic ratio because the embodiment is focused on aquasicrystal phase. That is, unless otherwise specified, “%” showing achemical composition in the following description means atomic %.

First, regarding the chemical composition of the plated-metal-layer, thenumerical ranges and why the numerical ranges are limited will bedescribed.

The plated-metal-layer of the plated steel sheet according to theembodiment contains Zn and Al as basic components, optional componentsas necessary, and Mg and impurities as a balance.

Zn (Zinc): 28.5% to 52%

In order to obtain a quasicrystal phase as a metallographic structure ofthe plated-metal-layer, the plated-metal-layer must contain Zn withinthe above range. Therefore, a Zn content in the plated-metal-layer isset to be 28.5% to 52%. The Zn content is determined based on a eutecticcomposition (Mg 72%-Zn 28%) of an equilibrium state diagram of a binaryMg—Zn system shown in FIG. 6, and the Zn content is higher than a Mg—Zneutectic composition. When a Mg content is higher than the eutecticcomposition and the Zn content is lower than the eutectic composition, aMg phase having poor corrosion resistance is mainly formed in themetallographic structure. Consequently, a corrosion resistance of theplated-metal-layer deteriorates. Therefore, the Zn content in theplated-metal-layer is set to be equal to or greater than 28.5%. Fromthis, a Zn phase can be dispersed in the plated-metal-layer as priority,in a proper production condition. A lower limit of the Zn content may be30% for a formation of the Zn phase in a further higher area fraction.The corrosion resistance is improved when the area fraction of the Znphase increases. However, when the Zn content is greater than 52%, acomposition balance of the plated-metal-layer is lost, intermetalliccompounds such as Mg₄Zn₇ and MgZn is formed in a large amount, and thequasicrystal phase is not formed. Consequently, the corrosion resistancedeteriorates. Therefore, the upper limit of the Zn content is set to be52%.

In order to further improve the corrosion resistance by preferablyforming the quasicrystal, the Zn content is preferably set to be equalto or greater than 33%. If the Zn content is equal to or greater than33%, a compositional range is established in which the Zn phase or thequasicrystal phase easily grows as a primary phase, and a Mg phase doesnot easily grow. That is, an amount (area fraction) of the quasicrystalphase in the plated-metal-layer can be increased, and an amount of theMg phase deteriorating corrosion resistance can be reduced as much aspossible. More preferably, the Zn content is set to be equal to orgreater than 35%. Generally, if the plated steel sheet is producedwithin the above compositional range by the production method accordingto the embodiment, the Mg phase practically does not exist.

Al (aluminum): 0.5% to 10%

Al is an element improving the corrosion resistance of a planar portionof the plated-metal-layer. Furthermore, Al is an element acceleratingthe formation of the quasicrystal phase. In order to obtain theseeffects, an Al content in the plated-metal-layer is set to be equal toor greater than 0.5%. Additionally, when the Al content is equal to orgreater than 0.5%, the Zn phase or an Al phase are formed in theplated-metal-layer in addition to the above described quasicrystalphase. The corrosion resistance of the plated-metal-layer is preferablyimproved by a formation of an eutectic structure of the Zn phase and theAl phase. The corrosion resistance is preferably further improved by ancoexistence of a three phase of the amorphous phase, the Zn phase, andthe Al phase. However, when the Al content is greater than 10%, acoarsening of a grain size of the Al phase drastically occurs in theplated-metal-layer, and the eutectic structure of the Zn phase and theAl phase is not formed. Additionally, the Zn phase is not formed.Consequently, the corrosion resistance deteriorates. Therefore, an upperlimit of the Al content in the plated-metal-layer is set to be 10%. Inorder to preferably control an average equivalent circle diameter of thequasicrystal phase, the Al content in the plated-metal-layer may be setto be less than 5%. When the average equivalent circle diameter of thequasicrystal phase becomes larger, although the corrosion resistance isnot affected, a conversion coating property becomes slightly inferior, acorrosion resistance under coating (a corrosion resistance aftercoating) is likely to deteriorate. In addition, it is preferable thatthe element Al is contained in the plated-metal-layer by forming a Fe—Alinterface alloy layer which will be described later.

It is preferable that a grain size of the quasicrystal phase in theplated-metal-layer is small, because an occurrence of red rust in aprocessed portion is suppressed. For example, in a case where thequasicrystal phase in a plated-metal-layer is coarse, theplated-metal-layer is likely to be partially peeled off from a steelsheet originating the quasicrystal phase in the plated-metal-layer, whena high deformation (drawing process, ironing process) or the like isapplied to the plated steel sheet. Red rust is likely to occur becausethe steel sheet is exposed at the peeled portion. In contrast, in a casewhere the quasicrystal phase in a plated-metal-layer is fine, theplated-metal-layer is not likely to be peeled off from a steel sheetoriginating the quasicrystal phase. If the plated-metal-layer is peeledoff from a steel sheet, the occurrence of red rust at the peeled portionis suppressed because an area of the peeled portion is small. Forexample, when the corrosion resistance of a cylindrical draw portion ofa plated steel sheet which is processed by a cylindrical drawing isevaluated, in a case where the average equivalent circle diameter of thequasicrystal phase is equal to or less than 1 μm, red rust is preferablysuppressed.

In order to more preferably form the quasicrystal phase in theplated-metal-layer, it is preferable to control the Zn content and theAl content as below. That is, the Zn content and the Al content,expressed in atomic %, in the chemical composition of theplated-metal-layer preferably satisfy 30%≤Zn+Al≤52%. When the Zn contentand the Al content satisfy the above condition, the quasicrystal phaseis formed in the plated-metal-layer at a preferred area fraction. It ispreferable that the Zn content and the Al content satisfy the aboveconditions, because then the quasicrystal phase is formed in theplated-metal-layer at an area fraction of 5% to 12% with respect to thetotal area of the plated-metal-layer. The technical reason is unclear.However, the formation of the quasicrystal phase at the aforementionedarea fraction is considered to be related to the facts that thequasicrystal phase in the embodiment has a crystal structure mainlycomposed of Zn and Mg, the formation of the quasicrystal phase isaccelerated by the substitution of Al with Zn, and there is an optimalvalue of the amount of Al substituted.

Mg (magnesium) is a main element which constitutes theplated-metal-layer similarly to Zn and Al and improves the sacrificialprotection. Furthermore, Mg is an important element accelerating theformation of the quasicrystal phase. In the embodiment, a content of Mgin the plated-metal-layer does not need to be particularly specified andis equal to the aforementioned balance minus a content of impurities.That is, the Mg content may be greater than 38% and less than 71%.However, the Mg content in the balance is preferably equal to or greaterthan 50%, and more preferably equal to or greater than 55%. In theembodiment, although the plated-metal-layer must contain Mg, in order toimprove the corrosion resistance, it is preferable to inhibit Mgcontained in the plated-metal-layer from being precipitated as a Mgphase in the plated-metal-layer. That is, because the Mg phasedeteriorates the corrosion resistance, it is preferable that Mgcontained in the plated-metal-layer is in the form of a quasicrystalphase or a constituent of other intermetallic compounds.

The plated-metal-layer of the plated steel sheet according to theembodiment contains impurities in addition to the aforementioned basiccomponents. Herein, the impurities mean elements such as C, N, O, P, S,and Cd that are mixed in from raw materials of steel and plated alloys,the production environment, or the like when the plated steel sheet isindustrially produced. Even if these elements are contained asimpurities in an amount of about 0.1% respectively, the aforementionedeffects are not impaired.

The plated-metal-layer of the plated steel sheet according to theembodiment may further contain, instead of a portion of Mg describedabove as a balance, at least one or more optional components selectedfrom Ca, Y, La, Ce, Si, Ti, Cr, Fe, Co, Ni, V, Nb, Cu, Sn, Mn, Sr, Sb,and Pb. The plated-metal-layer may contain these optional componentsaccording to the purpose. Therefore, the lower limit of the content ofthese optional components does not need to be limited and may be 0%.Even if these optional components are contained as impurities, theaforementioned effects are not impaired.

Ca (calcium): 0% to 3.5%

Y (yttrium): 0% to 3.5%

La (lanthanum): 0% to 3.5%

Ce (cerium): 0% to 3.5%

In order to improve workability of hot-dip plating, Ca, Y, La, and Cemay be contained in the plated-metal-layer. In a case where the platedsteel sheet according to the embodiment is produced, a highly oxidativehot-dip Mg alloy is held in the atmosphere as a plating bath. Therefore,it is preferable to take a certain measure to prevent the oxidation ofMg. Ca, Y, La, and Ce are more easily oxidized compared to Mg andprevent the oxidation of Mg in the bath by forming a stable oxide layeron the surface of the plating bath in a molten state. Accordingly, inthe plated-metal-layer, a Ca content may be set to be 0% to 3.5%, a Ycontent may be set to be 0% to 3.5%, a La content may be set to be 0% to3.5%, and a Ce content may be set to be 0% to 3.5%. More preferably, thelower limit and the upper limit of each of the Ca content, the Ycontent, the La content, and the Ce content may be set to be 0.3% and2.0% respectively.

It is preferable that the plated-metal-layer contain at least oneelement selected from Ca, Y, La, and Ce in an amount of equal to orgreater than 0.3% in total, because then the plating bath with a high Mgcontent can be held in the atmosphere without being oxidized. Incontrast, Ca, Y, La, and Ce are easily oxidized and negatively affectthe corrosion resistance in some cases. Therefore, the upper limit ofthe total content of Ca, Y, La, and Ce is preferably set to be 3.5%.That is, it is preferable that the Ca content, the Y content, the Lacontent, and the Ce content, expressed in atomic %, in the chemicalcomposition of the plated-metal-layer satisfy 0.3%≤Ca+Y+La+Ce≤3.5%.

In order to preferably generate the quasicrystal phase in theplated-metal-layer, it is preferable that the total content of Ca, Y,La, and Ce is set to be equal to or greater than 0.3% and equal to orless than 2.0%. Although these elements are considered to be substitutedwith Mg constituting the quasicrystal phase, in a case where theplated-metal-layer contains a large amount of these elements, theformation of the quasicrystal phase is hindered in some cases. If theplated-metal-layer contains these elements in an appropriate amount, theeffect of suppressing red rust of the quasicrystal phase is improved.Presumably, this effect may result from a fact that the elution timingof the quasicrystal phase affects retentivity of white rust. That is,presumably, after the quasicrystal phase in the plated-metal-layer iseluted, the aforementioned elements may be incorporated into the formedwhite rust, and accordingly, the rustproofness of white rust isimproved, and it takes a long time until red rust occurs due to thecorrosion of the base metal.

The obtained effects (antioxidation and the formation of thequasicrystal phase) described above become relatively strong when theplated-metal-layer contains Ca, La, Ce among the aforementionedelements. In contrast, it was revealed that the aforementioned effectsbrought about when the plated-metal-layer contains Y is weaker than theeffects brought about when the plated-metal-layer contain Ca, La, andCe. Presumably, this may be related to the fact that Ca, La, and Ce areelements that are more easily oxidized and have higher reactivitycompared to Y. When the chemical composition of the quasicrystal phaseis analyzed through Energy Dispersive X-ray Spectroscopy (EDX), Y is notdetected in many cases. Therefore, Y is presumed not to be easilyincorporated into the quasicrystal. In contrast, Ca, La, and Ce tend tobe detected from the quasicrystal phase at a high concentration comparedto the concentration thereof contained in the plated-metal-layer. Thatis, the plated-metal-layer does not necessarily contain Y. In a casewhere the plated-metal-layer does not contain Y, 0.3%≤Ca+La+Ce≤3.5% or0.3%≤Ca+La+Ce≤2.0% may be satisfied.

In a case where the atmosphere contacting the plating bath is purgedwith an inert gas (for example, Ar) or evacuated, that is, in a casewhere an oxygen blocking device is installed in the productionfacilities, Ca, Y, La, and Ce are not necessarily added.

The total content of Al, Ca, La, Y, and Ce is preferably controlled asbelow. That is, the Al content, the Ca content, the La content, the Ycontent, and the Ce content, expressed in atomic %, in the chemicalcomposition of the plated-metal-layer preferably satisfy0.5%≤Al+Ca+La+Y+Ce<6%, and more preferably satisfy0.5%≤Al+Ca+La+Y+Ce<5.5%. If the total content of Al, Ca, La, Y, and Cesatisfies the aforementioned conditions, a quasicrystal phase having apreferred average equivalent circle diameter is formed in theplated-metal-layer. If the aforementioned conditions are satisfied, theaverage equivalent circle diameter of the quasicrystal phase can becontrolled and become equal to or smaller than 0.6 μm. Furthermore, itis possible to improve powdering properties (peeling resistance againsta compressive stress) of the plated layer. Presumably, Ca, La, Y, Ce, orthe like added in a trace amount in addition to Al may be precipitatedin the grain boundary of the quasicrystal phase, and hence the grainboundary may be strengthened.

Si (silicon): 0% to 0.5%

Ti (titanium): 0% to 0.5%

Cr (chromium): 0% to 0.5%

The plated-metal-layer may contain Si, Ti, and Cr, as necessary, suchthat the quasicrystal phase is preferably formed. If theplated-metal-layer contains Si, Ti, and Cr in a trace amount, thequasicrystal phase is easily formed or the structure of the quasicrystalphase is stabilized. It is considered that Si may become an origin(nuclear) of the formation of the quasicrystal phase by means of formingfine Mg₂Si by being bonded to Mg, and Ti and Cr, which exhibit poorreactivity with respect to Mg, may become the origin of the formation ofthe quasicrystal phase by means of forming a fine metallic phase.Furthermore, generally, the formation of the quasicrystal phase isaffected by a cooling rate at the time of production. If theplated-metal-layer contains Si, Ti, and Cr, the cooling rate tends tobecome less dependent on the formation of the quasicrystal phase.Therefore, in the plated-metal-layer, a Si content may be set to be 0%to 0.5%, a Ti content may be set to be 0% to 0.5%, and a Cr content maybe set to be 0% to 0.5%. More preferably, the lower limit and the upperlimit of each of the Si content, the Ti content, and the Cr content maybe set to be 0.005% and 0.1% respectively.

It is preferable that the plated-metal-layer contains at least oneelement selected from Si. Ti, and Cr in an amount of 0.005% to 0.5% intotal, because then the structure of the quasicrystal is furtherstabilized. That is, the Si content, the Ti content, and the Cr content,expressed in atomic %, in the chemical composition of theplated-metal-layer preferably satisfy 0.005%≤Si+Ti+Cr≤0.5%. Furthermore,if these elements are contained in an appropriate amount, thequasicrystal is preferably formed finely and in a large amount, and thecorrosion resistance of the surface of the plated-metal-layer isimproved. In addition, the corrosion resistance in a humid environmentis improved, and the occurrence of white rust is inhibited.

Co (cobalt): 0% to 0.5%

Ni (nickel): 0% to 0.5%

V (vanadium): 0% to 0.5%

Nb (niobium): 0% to 0.5%

Co, Ni, V, and Nb have the same effects as those of Si, Ti, and Crdescribed above. In order to obtain the aforementioned effects, a Cocontent may be set to be 0% to 0.5%, a Ni content may be set to be 0% to0.5%, a V content may be set to be 0% to 0.5%, and a Nb content may beset to be 0% to 0.5%. More preferably, the lower limit and the upperlimit of each of the Co content, the Ni content, the V content, and theNb content may be set to be 0.05% and 0.1% respectively. Here, thecorrosion resistance improving effects of these elements are weaker thanthat of Si, Ti, and Cr.

In some cases, the elements constituting a steel sheet, which is a basemetal, are mixed into the plated-metal-layer from the steel sheet.Particularly, during hot-dip plating, due to the mutual diffusion ofelements in which the elements are diffused from the steel sheet to theplated-metal-layer and from the plated-metal-layer to the steel sheet,the adherence is improved. Therefore, the plated-metal-layer contains acertain amount of Fe (iron) in some cases. For example, Fe is containedin an amount of around 2% in the entire chemical composition of theplated-metal-layer in some cases. However, Fe diffused to theplated-metal-layer frequently forms an intermetallic compound byreacting with Al and Zn in the vicinity of the interface between thesteel sheet and the plated-metal-layer. Therefore, Fe is less likely toaffect the corrosion resistance of the plated-metal-layer. Consequently,an Fe content in the plated-metal-layer may be set to be 0% to 2%.Similarly, the elements constituting the steel sheet that have beendiffused to the plated-metal-layer (elements other than the elementsdescribed above in the embodiment that have been diffused to theplated-metal-layer from the steel sheet) are less likely to affect thecorrosion resistance of the plated-metal-layer.

Cu (copper): 0% to 0.5%

Sn (tin): 0% to 0.5%

In order to improve the adherence between the steel sheet and theplated-metal-layer, the steel sheet having not yet been subjected to ahot-dip plating process is preliminarily plated with Ni, Cu, Sn, or thelike in some cases. In a case where the plated steel sheet is producedusing the preliminarily plated steel sheet, the plated-metal-layercontains the aforementioned elements approximately in an amount of up to0.5% in some cases. Among Ni, Cu, and Sn, Cu and Sn do not have theaforementioned effects that Ni has. However, even if Cu and Sn arecontained in an amount of about 0.5% in the plated-metal-layer, they areless likely to affect the quasicrystal formation behavior or thecorrosion resistance of the plated-metal-layer. Therefore, in theplated-metal-layer, a Cu content may be set to be 0% to 0.5%, and a Sncontent may be set to be 0% to 0.5%. More preferably, the lower limitand the upper limit of each of the Cu content and the Sn content may beset to be 0.005% and 0.4% respectively.

Mn (manganese): 0% to 0.2%

In recent years, as a steel sheet which is a base metal of a platedsteel sheet, high tensile strength steel (high strength steel) has beenused. In a case where a plated steel sheet is produced using the hightensile strength steel, the elements such as Si and Mn contained in thehigh tensile strength steel are diffused in the plated-metal-layer insome cases. Between Si and Mn, Mn does not have the aforementionedeffects that Si has. However, even if the Mn is contained in an amountof about 0.2% in the plated-metal-layer, the elements is not likely toaffect the quasicrystal formation behavior or the corrosion resistanceof the plated-metal-layer. Therefore, a Mn content in theplated-metal-layer may be set to be 0% to 0.2%. More preferably, thelower limit and the upper limit of the Mn content may be set to be0.005% and 0.1% respectively.

Sr (strontium): 0% to 0.5%

Sb (antimony): 0% to 0.5%

Pb (lead): 0% to 0.5%

Sr, Sb, and Pb are elements improving the appearance of plating andeffective for improving antiglare properties. In order to obtain theeffect, in the plated-metal-layer, a Sr content may be set to be 0% to0.5%, a Sb content may be set to be 0% to 0.5%, and a Pb content may beset to be 0% to 0.5%. In a case where the Sr content, the Sb content,and the Pb content are within the above range, the elements practicallydo not affect the corrosion resistance. More preferably, the lower limitand the upper limit of each of the Sr content, the Sb content, and thePb content may be set to be 0.005% and 0.4% respectively.

The aforementioned chemical composition of the plated-metal-layer ismeasured using Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), or thelike. The plated steel sheet is dipped in 10% hydrochloric acid, towhich an inhibitor added, for about 1 minute such that theplated-metal-layer portion is peeled off, thereby preparing a solutionin which the plated-metal-layer is dissolved. By analyzing the solutionthrough ICP-AES, ICP-MS, or the like, the average chemical compositionof the whole plated-metal-layer is obtained.

During the hot-dip plating, a plated-metal-layer having substantiallythe same chemical composition as the chemical composition of the hot-dipplating bath is formed. Therefore, regarding the elements that undergomutual diffusion between the steel sheet and the plated-metal-layer to anegligible extent, the chemical composition of the plating bath may bemeasured, and the measured value may be adopted as the chemicalcomposition of the plated-metal-layer. From the plating bath, a smallingot is collected, drill dust is then collected, and a solutionobtained by dissolving the drill dust in an acid is prepared. Byanalyzing the solution through ICP or the like, the chemical compositionof the plating bath is obtained. The measured value of the chemicalcomposition of the plating bath may be used as the chemical compositionof the plated-metal-layer.

Next, the metallographic structure of the plated-metal-layer will bedescribed.

The plated-metal-layer of the plated steel sheet according to theembodiment contains a quasicrystal phase as a metallographic structure.The quasicrystal phase is defined as a quasicrystal phase in which thecontents of Mg, Zn, and Al, expressed in atomic %, contained in thequasicrystal phase satisfy 0.5≤Mg/(Zn+Al)≤0.83. That is, thequasicrystal phase is defined as a quasicrystal phase in which Mg:(Zn+Al), a ratio of the number of Mg atoms to the total number of Zn atomsand Al atoms, is 3:6 to 5:6. Theoretically, the ratio of Mg:(Zn+Al) isconsidered to be 4:6. The chemical composition of the quasicrystal phaseis preferably calculated by quantitative analysis based on TransmissionElectron Microscope-Energy Dispersive X-ray Spectroscopy (TEM-EDX) or byquantitative analysis based on Electron Probe Micro-Analyzer (EPMA)mapping. It is not easy to define the quasicrystal by using an accuratechemical formula just as an intermetallic compound. This is because arepeating lattice unit of the quasicrystal phase cannot be definedunlike a unit lattice of a crystal, and it is difficult to identify theatomic position of Zn and Mg.

In the embodiment, an average equivalent circle diameter of thequasicrystal phase contained in the plated-metal-layer is equal to orlarger than 0.01 μm and equal to or smaller than 1 μm. As thequasicrystal phase, by a micrograph and an electron diffraction patternobtained by TEM, the quasicrystal phase having an average equivalentcircle diameter of about 0.01 μm or larger can be identified. It isconsidered that the plated-metal-layer may include the quasicrystalphase having an equivalent circle diameter of less than 0.01 μm, fromthe above described reason, a lower limit of the equivalent circlediameter of the quasicrystal phase is set to be 0.01 μm. Additionally,an upper limit of the equivalent circle diameter of the quasicrystalphase is not particularly limited, according to a metallographicstructure of the plated-metal-layer which is described below, it ispreferable that the upper limit of the equivalent circle diameter of thequasicrystal phase is set to be 1 μm. For further improving thecorrosion resistance under coating (the corrosion resistance aftercoating), it is preferable that the upper limit of the equivalent circlediameter of the quasicrystal phase is set to be 0.8 μm, and it is morepreferable that the upper limit of the equivalent circle diameter of thequasicrystal phase is set to be 0.6 μm. In addition, for furtherimproving the corrosion resistance after an ironing process is applied,it is preferable that the upper limit of the average equivalent circlediameter of the quasicrystal phase is set to be 0.5 μm, and it is morepreferable that the upper limit of the equivalent circle diameter of thequasicrystal phase is set to be 0.4 μm.

It is preferable that in a case where a cross section whose cuttingdirection is parallel to a thickness direction of the plated-metal-layeris viewed, the metallographic structure of the plated-metal-layer is abimodal structure which consists of a coarse domain composed of a grainhaving an equivalent circle diameter of larger than 0.2 μm and a finedomain composed of a grain having an equivalent circle diameter 0.2 μmor smaller. An upper limit of the equivalent circle diameter of thegrain included in the coarse domain and a lower limit of the equivalentcircle diameter of the grain included in the fine domain are notparticularly limited. However, when the coarse domain becomes larger toomuch, the dispersion of the metallographic structure becomes uniformly.Therefore, the upper limit of the equivalent circle diameter of thegrain included in the coarse domain may be set to be 10 μm, 5 μm, 2 μm,or 1 μm. Additionally, the lower limit of the fine domain may be morethan 0 μm or 0.01 μm as necessary.

When the average equivalent circle diameter of the quasicrystal phase islarger than 0.2 μm and equal to or less than 1 μm, in the abovedescribed bimodal structure, it is preferable that the coarse domainincludes at least one or more kinds of phase selected from thequasicrystal phase, the Zn phase, the Al phase, and the MgZn phase, andthe fine domain includes at least one or more kinds of phase selectedfrom a Mg₅₁Zn₂₀ phase, the Zn phase, the amorphous phase, and aMg₃₂(Zn,Al)₄₉ phase.

When the average equivalent circle diameter of the quasicrystal phase isequal to or larger than 0.01 μm and equal to or less than 0.2 μm, in theabove described bimodal structure, it is preferable that the coarsedomain includes at least one or more kinds of phase selected from the Znphase, the Al phase, and the MgZn phase, and the fine domain includes atleast one or more kinds of phase selected from the quasicrystal phase, aMg₅₁Zn₂₀ phase, the Zn phase, the amorphous phase, and a Mg₃₂(Zn,Al)₄₉phase.

The Zn phase and the Al phase included in the plated-metal-layer mayform an eutectic structure. In a case where the Zn phase and the Alphase form an eutectic structure, it is preferable that the averageequivalent circle diameter of a block size of the eutectic structure islarger than 0.2 μm.

If the metallographic structure of the plated-metal-layer is controlledto become a bimodal structure consisting of the coarse domain and thefine domain as described above, the corrosion resistance is preferablyimproved.

Generally, a bimodal structure means a structure in which a frequencydistribution such as the equivalent circle diameter of the grainincluded in the metallographic structure becomes double-peakdistribution. In the plated steel sheet according to the embodiment, itis preferable that the frequency distribution of the equivalent circlediameter of the grain included in the metallographic structure of theplated-metal-layer is a double-peak distribution. Here, in the platedsteel sheet according to the embodiment, the frequency distribution isnot necessarily a double-peak distribution, and the aforementionedeffects are obtained even if the frequency distribution is a broaddistribution. That is, in the embodiment, the bimodal structure meansthat the frequency distribution of the equivalent circle diameter of thegrain included in the metallographic structure of the plated-metal-layeris not a normal distribution, and the metallographic structure of theplated-metal-layer consists of the fine domain composed of a grainhaving an equivalent circle diameter of 0.2 μm or smaller and the coarsedomain composed of a grain having an equivalent circle diameter oflarger than 0.2 μm.

As described above, the average equivalent circle diameter of thequasicrystal phase contained in the metallographic structure of theplated-metal-layer of the plated steel sheet according to the embodimentis equal to or larger than 0.01 μm and equal to or smaller than 1 μm.That is, in a case where each grain of the quasicrystal phase isseparately considered, a quasicrystal phase having an equivalent circlediameter of equal to or smaller than 0.2 μm and a quasicrystal phasehaving an equivalent circle diameter of larger than 0.2 μm are containedin the metallographic structure of the plated-metal-layer. Here, in acase where the average equivalent circle diameter of the quasicrystalphase is larger than 0.2 μm and equal to or smaller than 1 μm, thequasicrystal phase is mainly included in the coarse domain of theplated-metal-layer. Additionally, in a case where the average equivalentcircle diameter of the quasicrystal phase is equal to or larger than0.01 μm and equal to or smaller than 0.2 μm, the quasicrystal phase ismainly included in the fine domain of the plated-metal-layer.

The average equivalent circle diameter of a constituent phase includedin the coarse domain, for example, the Al phase, the Zn phase, or thelike, is preferably set to be larger than 0.2 μm. In this case, grainsof the aforementioned phases having an equivalent circle diameter ofequal to or smaller than 0.2 μm and grains of the aforementioned phaseshaving an equivalent circle diameter of larger than 0.2 μm are includedin the metallographic structure of the plated-metal-layer. Theaforementioned grains are mainly included in the coarse domain.Additionally, in a case where the Al phase and the Zn phase form theeutectic structure, it is preferable that an average equivalent circlediameter of a block size of the eutectic structure is larger than 0.2μm. In this case, a block having an equivalent circle diameter of equalto or smaller than 0.2 μm and a block having an equivalent circlediameter of larger than 0.2 μm are included in the metallographicstructure of the plated-metal-layer. The eutectic structure of the Znphase and the Al phase is mainly included in the coarse domain. An upperlimit of the average equivalent circle diameter of the constituent phaseor the eutectic structure included in the coarse domain is notparticularly limited. The upper limit may be set to be 10 μm, 5 μm, 2μm, or 1 μm as necessary.

The average equivalent circle diameter of a constituent phase includedin the fine domain, for example, the Mg₅₁Zn₂₀ phase, the amorphousphase, the Mg₃₂(Zn,Al)₄₉ phase or the like, is preferably set to be 0.01μm to 0.2 μm. In this case, grains of the aforementioned phases havingan equivalent circle diameter of equal to or smaller than 0.2 μm andgrains of the aforementioned phases having an equivalent circle diameterof larger than 0.2 μm are included in the metallographic structure ofthe plated-metal-layer. The aforementioned grains are mainly included inthe fine domain.

The average equivalent circle diameter of the Zn phase included in themetallographic structure is not particularly limited. The upper limitmay be set to be 10 μm, 5 μm, 2 μm, or 1 μm as necessary. The lowerlimit may be set to be more than 0 μm or 0.01 μm as necessary. That is,the Zn phase may be mainly included in the coarse domain, or the Znphase may be mainly included in the fine domain.

FIG. 1 is an electron micrograph of a plated steel sheet according tothe embodiment, which is a metallographic micrograph obtained byobserving a cross section whose cutting direction is parallel to athickness direction of the plated steel sheet. This cross-sectionalimage is a backscattered electron compositional image (COMPO image)obtained by observation using a Scanning Electron Microscope (SEM). InFIG. 1, 1 indicates a steel sheet, and 2 indicates a plated-metal-layer.Furthermore, in FIG. 1, 2 a indicates a coarse domain, and 2 b indicatesa fine domain. At least one or more kinds of phase among thequasicrystal phase, the Zn phase, the Al phase, and the MgZn phase areincluded in the coarse domain 2 a, and at least one or more kinds ofphase among the quasicrystal phase, the Mg₅₁Zn₂₀ phase, the Zn phase,the amorphous phase, and the Mg₃₂(Zn,Al)₄₉ phase are included in thefine domain 2 b. FIG. 1 shows that the metallographic structure of theplated-metal-layer is a bimodal structure.

In a strict sense, fine intermetallic compounds or metal phases havingan equivalent circle diameter of 0.2 μm or smaller are diffused in thecoarse domain 2 a in some cases. However, fine grains present in thecoarse domain 2 a are not regarded as being the fine domain 2 b. In theembodiment, the fine domain 2 b refers to a domain in which a pluralityof fine grains having an equivalent circle diameter 0.2 μm or smaller iscontinuously piled up and which is found to be an equivalent area whenbeing observed at an SEM level.

FIGS. 2 and 3 are electron micrographs of the plated-metal-layer of theplated steel sheet according to the same embodiment, which is ametallographic micrograph obtained by observing the cross section whosecutting direction is parallel to the thickness direction of the platedsteel sheet. This cross-sectional image is obtained by observation usingTEM and is a bright field image. FIG. 2 is a metallographic micrographof a vicinity of a surface of the plated-metal-layer, and FIG. 3 is ametallographic micrograph of a vicinity of an interface between theplated-metal-layer and the steel sheet. In FIGS. 2 and 3, 2 a indicatesa coarse domain, and 2 b indicates a fine domain. Similarly to FIG. 1,FIGS. 2 and 3 show that the metallographic structure of theplated-metal-layer is a bimodal structure.

FIG. 4A is an electron diffraction pattern obtained from a local area 2a 1 in the coarse domain 2 a shown in FIG. 2. FIG. 4B is an electrondiffraction pattern obtained from a local area 2 b 1 in the fine domain2 b shown in FIG. 2. FIG. 4A shows an electron diffraction pattern of aradial regular decagon resulting from an icosahedron structure. Theelectron diffraction pattern shown in FIG. 4A is obtained from only aquasicrystal and cannot be obtained from any other crystal structures.From the electron diffraction pattern shown in FIG. 4A, it can beconfirmed that the quasicrystal phase is included in the coarse domain 2a. FIG. 4B shows an electron diffraction pattern resulting from theMg₅₁Zn₂₀ phase. From the electron diffraction pattern shown in FIG. 4B,it can be confirmed that the Mg₅₁Zn₂₀ phase is included in the finedomain 2 b.

FIG. 5A is an electron diffraction pattern obtained from a local area 2a 2 in the coarse domain 2 a shown in FIG. 3. FIG. 5B is an electrondiffraction pattern obtained from a local area 2 b 2 in the fine domain2 b shown in FIG. 3. From the electron diffraction pattern shown in FIG.5A, it can be confirmed that the MgZn phase is included in the coarsedomain 2 a. From the electron diffraction pattern shown in FIG. 5B, itcan be confirmed that the Zn phase is included in the fine domain 2 b.Furthermore, it was confirmed that the Zn phase or the Al phase or thelike are included in the coarse domain 2 a and the quasicrystal phase,the amorphous phase and the Mg₃₂(Zn,Al)₄₉ phase are included in the finedomain 2 b in some cases, although such cases are not shown in thedrawing.

Here, the quasicrystal which has the average equivalent circle diameterof equal to or less than 1 μm is very fine, and hence the diffractionpatterns obtained from multiple quasicrystals may be superposedaccording to an irradiation position of an electron beam. In this case,a clear electron diffraction pattern of a radial regular decagon may notbe obtained, and a diffraction pattern that is similar to a halo patternpeculiar to the amorphous phase. Therefore, it should be needed to takecare to identify the quasicrystal phase.

In the fine domain 2 b, a large amount of Mg₅₁Zn₂₀ is observed in a casewhere the Mg content is high, and a large amount of Mg₃₂(Zn,Al)₄₉ phaseis observed in a case where the Mg content is low. The presence ofintermetallic compounds or metal phases such as the Zn phase, the Alphase, the MgZn phase, the Mg₅₁Zn₂₀ phase, the Mg₃₂(Zn,Al)₄₉ phase, andthe amorphous phase, or the like, can be confirmed using an electrondiffraction pattern obtained by TEM as descried above or confirmed usingan X-Ray Diffractometer (XRD).

The Mg₅₁Zn₂₀ phase is defined as a constituent phase which can beidentified by a JCPDS card: PDF#00-008-0269, #00-065-4290, or anon-patent document “Journal of solid state chemistry 36, 225-233(1981), Yamato et al.” Furthermore, the Mg₃₂(Zn,Al)₄₉ phase is definedas a constituent phase which can be identified by JCPDS card:PDF#00-019-0029 or #00-039-0951.

The chemical composition of the aforementioned intermetallic compoundsor metallic phases can be quantitatively analyzed in a simple mannerthrough TEM-EDX or EPMA. From the result of the quantitative analysis,whether each grain in a constituent phase is one of the quasicrystalphase, the Mg₅₁Zn₂₀ phase, the Mg₃₂(Zn,Al)₄₉ phase, the Mg₄Zn₇ phase,the MgZn phase, the Mg phase, and the Zn phase or other phases can beidentified in a simple manner.

The non-patent document “Journal of solid state chemistry 36, 225-233(1981), Yamato et al.” reports that Mg₅₁Zn₂₀ has a unit lattice close toa cubical crystal and has an atomic structure in which an icosahedron isformed in the unit lattice. The unit lattice of Mg₅₁Zn₂₀ is differentfrom the icosahedron structure of the quasicrystal, and accordingly, ina strict sense, Mg₅₁Zn₂₀ is a phase different from the quasicrystal.However, it is considered that, because the crystal structure ofMg₅₁Zn₂₀ is similar to that of the quasicrystal, the Mg₅₁Zn₂₀ phase mayaffect the formation of the quasicrystal phase. Mg₃₂(Zn,Al)₄₉ is alsocalled a Frank-Kasper phase and has a complicated atomic configuration(rhombic triacontahedron). Presumably, the Mg₃₂(Zn,Al)₄₉ phase may alsobe closely related to the formation of the quasicrystal phase similarlyto the Mg₅₁Zn₂₀ phase.

The amorphous phase, the quasicrystal phase, and the Mg₅₁Zn₂₀ phaseincluded in the fine domain 2 b in some cases are different in an atomicarrangement (crystal structure), however, there is almost no differencein these phases in regard to a chemical composition represented by aratio between Zn and Mg. It was confirmed that there is almost nodifference in the chemical compositions of the quasicrystal phase, theMg₅₁Zn₂₀ phase, and the amorphous phase through TEM-EDX mapping of thefine domain 2 b, although it is not shown in the drawing. It isdetermined that the quasicrystal phase, the Mg₅₁Zn₂₀ phase, and theamorphous phase are non-equilibrium phases which is formed by anaccelerated cooling before an equilibrium phase is precipitated, in aproduction of a plated steel sheet.

The corrosion resistance of the constituent phases of theplated-metal-layer tends to be excellent in order of the quasicrystalphase>the Mg₃₂(Zn,Al)₄₉ phase>the Mg₅₁Zn₂₀ phase>the MgZn phase>the Alphase>the Zn phase>the amorphous phase>>the Mg phase. In a case wherethese constituent phase are mixed together, increasing a fraction of thephase having high corrosion resistance and uniformly dispersing thephase favors the corrosion resistance of the plated-metal-layer.

Here, in a case where various metallic phases or intermetallic compoundscoexist in the plated-metal-layer, due to the formation of a couplingcell, the corrosion resistance further deteriorates than in a case wherea single phase exists in the plated-metal-layer. Generally, if aplurality of phases is mixed into the plated-metal-layer, portions thatare noble and less noble in terms of electric energy formed in theplated-metal-layer, and hence a coupling cell reaction occurs. The lessnoble portions corrode first, and hence the corrosion resistancedeteriorates. Here, in the plated steel sheet according to theembodiment, in a case where the plated-metal-layer has theaforementioned bimodal structure, the deterioration of corrosionresistance resulting from the formation of the coupling cell ispractically not observed and negligible, and rather, the corrosionresistance is markedly improved because the plated-metal-layer containsthe quasicrystal.

Furthermore, the average equivalent circle diameter of each of theconstituent phase included in the coarse domain may be set to be equalto or less than 2 μm, preferably it may be set to be equal to or lessthan 1 μm. In this case, for example, an existing Zn-based conversioncoating can be applied. In a Zn-based phosphate treatment which is usedas a paint undercoat treatment of a metal, generally, when the grainsize of the constituent phases included in the coarse domain isincreased, a phosphoric acid crystal is less likely to grow on thecoarse domain. Then, this domain becomes a lack of hiding (a domainwhere a conversion crystal is not formed, because a conversion coatingproperty is poor), and the corrosion resistance after coating maydeteriorate significantly. In contrast, when the average equivalentcircle diameter of each of the constituent phase included in the coarsedomain is set to be equal to or less than 2 μm, preferably it is set tobe equal to or less than 1 μm, a phosphoric acid crystal can preferablygrow.

Generally, an intermetallic compound phase or the amorphous phase havepoor plastic deformation properties. If a fraction of a coarseconstituent phase having poor plastic workability is reduced, only finecracks occur in the plated-metal-layer at the time of processing theplated steel sheet. Accordingly, an exposed area of the steel sheet(base metal) is reduced, and the corrosion resistance is preferablyimproved. Furthermore, because the peeling of the plated-metal-layer isinhibited, it takes a long time until red rust occurs in the processedportion, and hence the corrosion resistance is preferably improved.

The quasicrystal phase is a non-equilibrium phase and thermallyunstable. Therefore, if exposed to a high temperature environment with atemperature of around 250° C. to 330° C. for a long period of time, thequasicrystal phase undergoes phase decomposition, and hence the Mg phasehaving poor corrosion resistance is formed in addition to the Mg₅₁Zn₂₀phase in some cases. Consequently, the corrosion resistance as theoverall plated steel sheet is likely to deteriorate. Care is required ina case where the plated steel sheet is used in a high temperatureenvironment.

As described above, the Zn phase and the Al phase may form the eutecticstructure. In the embodiment, the Zn phase is a phase containing morethan 95% of Zn, an element such as Mg, Al, and Ca constituting theplated-metal-layer is dissolved in the Zn phase less than 5%.Furthermore, the Al phase is a phase containing more than 33% of Al, Znis mainly contained in the Al phase in addition to Al, and a smallamount of an element constituting the plated-metal-layer, such as Mg orCa is dissolved in the Al phase. Further, the eutectic structure of theZn phase and the Al phase (Zn—Al eutectic) refers to an eutecticstructure constituted with mixed phase of the aforementioned Zn phaseand the aforementioned Al phase.

In the plated-metal-layer of the plated steel sheet according to theembodiment, an area fraction of the coarse domain in the metallographicstructure of the entirety of the plated-metal-layer (area of coarsedomain/area of plated-metal-layer) is preferably 5% to 50%, and an areafraction of the fine domain in the metallographic structure of theentirety of the plated-metal-layer (area of fine domain/area ofplated-metal-layer) is preferably 50% to 95%. If the above conditionsare satisfied, the corrosion resistance of the plated-metal-layer isfurther improved. More preferably, an area fraction of the coarse domainin the metallographic structure of the entirety of theplated-metal-layer may be 5% to 20%, and an area fraction of the coarsedomain may be 5% to 10%.

In the plated-metal-layer of the plated steel sheet according to theembodiment, in a case where the average equivalent circle diameter islarger than 2 μm and equal to or smaller than 1 μm, an area fraction ofthe quasicrystal phase included in the coarse domain is preferably 80%to less than 100% as compared with the coarse domain (area ofquasicrystal phase in coarse domain/area of coarse domain), and an areafraction in total of the Mg₅₁Zn₂₀ phase, the Zn phase, the amorphousphase, and the Mg₃₂(Zn,Al)₄₉ phase included in the fine domain ispreferably 80% to less than 100% as compared with the fine domain (totalarea of respective constituent phases in fine domain/area of finedomain). In contrast, in a case where the average equivalent circlediameter is equal to or larger than 0.01 μm and equal to or smaller than0.2 μm, an area fraction in total of the Zn phase, the Al phase, and theMgZn phase included in the coarse domain is preferably 80% to less than100% as compared with the coarse domain (area of quasicrystal phase incoarse domain/area of coarse domain), and an area fraction of thequasicrystal phase included in the fine domain is preferably more than0% to less than 10% as compared with the fine domain (total area ofrespective constituent phases in fine domain/area of fine domain). Whenthe above conditions are satisfied, the corrosion resistance of theplated-metal-layer is further improved. The balance of the coarse domainand the balance of the fine domain include an intermetallic compound ora metallic phase other than the above in some cases, but even in thesecases, the effects of the embodiment are not impaired.

It is preferable that the metallographic structure of theplated-metal-layer in the plated steel sheet according to the embodimentdoes not contain the Mg phase. The Mg phase contained in theplated-metal-layer deteriorates the corrosion resistance in both thecoarse domain and the fine domain. Therefore, it is preferable tosuppress the precipitation of the Mg phase as much as possible. Whetheror not the Mg phase exists may be determined and confirmed throughTEM-EDX, SEM-EDX, XRD, or the like. For example, in a case where adiffraction intensity from a (110) surface of the Mg phase is equal toor less than 1% of a diffraction intensity at a diffraction angle(2θ=36.496°) of the Mg₅₁Zn₂₀ phase (or Mg₇Zn₃ phase) in an XRDdiffraction pattern, it can be said that the metallographic structure ofthe plated-metal-layer does not contain the Mg phase. Likewise, in acase where a number fraction of grains of the Mg phase is equal to orless than 3% when 100 or more grains are randomly sampled in a TEMdiffraction pattern, it can be said that the metallographic structure ofthe plated-metal-layer does not contain the Mg phase. The numberfraction of grains of the Mg phase is more preferably less than 2%, andmost preferably less than 1%.

In the plated-metal-layer, the Mg phase is easily formed as a primaryphase at a temperature immediately below the melting point. Whether theMg phase will be formed as a primary phase generally depends on thechemical composition of the plated-metal-layer and the productionconditions. In a case where the Mg content is higher than in a eutecticcomposition (Mg 72%-Zn 28%) of an equilibrium state diagram of a binaryMg—Zn system, the Mg phase is likely to be crystallized as a primaryphase. In contrast, in a case where the Mg content is lower than theabove, in principle, the Mg phase is less likely to be crystallized as aprimary phase. The production process according to the embodiment is aprocess for forming a quasicrystal as a primary phase. Therefore, if theMg content is higher than in the eutectic composition, it is extremelydifficult for the Mg phase to be formed, and even if the formation ofthe Mg phase could be confirmed, the Mg phase is less likely to presentas a main phase. The grain of the Mg phase is present at a numberfraction of about up to 3%. The present inventors confirmed that whenthe Zn content is 28.5% or greater, a proportion of the grain of the Mgphase in grains contained in the metallographic structure of theplated-metal-layer tends to be less than 2% in terms of a numberfraction. Furthermore, when the Zn content is 33% or greater, aproportion of the grain of the Mg phase in the grains contained in themetallographic structure of the plated-metal-layer tends to be less than1% in terms of a number fraction. If the Mg phase is present in theplated-metal-layer, the surface of the plated-metal-layer turns blackwith the passage of time particularly in a humid environment, and hencethe appearance of the plating becomes defective in some cases. In thisrespect, it is preferable to avoid mixing of the Mg phase into thesurface layer of the plated-metal-layer in particular. By storing theplated steel sheet in a thermohygrostat tank for a certain period oftime, the occurrence of appearance defectiveness, a phenomenon in whichthe surface of the plated-metal-layer turns black, can be determined.

Regarding the plated-metal-layer of the plated steel sheet according tothe embodiment, when a cross section whose cutting direction is parallelto a thickness direction of the plated-metal-layer is viewed and when athickness of the plated-metal-layer in the thickness direction isregarded as D in a unit of μm, an area from a surface of theplated-metal-layer toward the steel sheet in the thickness direction to0.05×D is regarded as an outermost area of the plated-metal-layer, andan area from the interface between the steel sheet and theplated-metal-layer toward the plated-metal-layer in the thicknessdirection to 0.05×D is regarded as an innermost area of theplated-metal-layer, an area fraction of the coarse domain in theoutermost area of the plated-metal-layer (area of coarse domain inoutermost area of plated-metal-layer/area of outermost area ofplated-metal-layer) is preferably 7% to less than 100% and an areafraction of the coarse domain in the innermost area of theplated-metal-layer (area of coarse domain in innermost area ofplated-metal-layer/area of innermost area of plated-metal-layer) ispreferably 7% to less than 100%. Furthermore, when an area except forthe outermost area and the innermost area in the plated-metal-layer isregarded as a main-body area of the plated-metal-layer, an area fractionof the fine domain in the main-body area of the plated-metal-layer (areaof fine domain in main-body area of plated-metal-layer/area of main-bodyarea of plated-metal-layer) is preferably 50% to less than 100%. If theabove conditions are satisfied, the constituent phases contained in theplated-metal-layer are preferably arranged, and hence the corrosionresistance of the plated-metal-layer is further improved. In addition,the adherence of the plated-metal-layer tends to be improved. In a casewhere the grains in the coarse domain are present in a position thatextends across the outermost area and the innermost area of theplated-metal-layer, or in a case where the grains in the coarse domainare present in a position that extends across the innermost area and themain-body area of the plated-metal-layer, the aforementioned areafraction may be calculated using the area of the grains included in theoutermost area or deep area of the plated-metal-layer. Likewise, in acase where the grains in the fine domain are present in a position thatextends across the outermost area and innermost area of theplated-metal-layer, or in a case where the grains in the fine domain arepresent in a position that extends across the innermost area and themain-body area of the plated-metal-layer, the aforementioned areafraction may be calculated using the area of grains included in themain-body area of the plated-metal-layer.

The plated steel sheet according to the embodiment preferably furtherhas a Fe—Al containing alloy layer (an alloy layer containing Fe—Al).The Fe—Al containing alloy layer is preferably arranged between thesteel sheet and the plated-metal-layer and preferably contains at leastone or more kinds of compound between Fe₅Al₂ and Al_(3.2)Fe, and athickness of the Fe—Al containing alloy layer in a thickness directionthereof is preferably 10 nm to 1,000 nm. If the Fe—Al containing alloylayer satisfying the above conditions is arranged in the interfacebetween the steel sheet and the plated-metal-layer, peeling of theplated-metal-layer is preferably inhibited. Furthermore, if the Fe—Alcontaining alloy layer is formed, the adherence of theplated-metal-layer tends to be improved.

The thickness D of the plated-metal-layer of the plated steel sheetaccording to the embodiment is not particularly limited, and may becontrolled as necessary. Generally, the thickness D is set to be 35 μmsmaller in many cases.

The metallographic structure of the plated-metal-layer is observed asbelow. A sample is collected by cutting the plated steel sheet such thatthe cross section whose cutting direction is parallel to the thicknessdirection of the plated steel sheet is observed. The cross section ispolished or processed by using a Cross Section Polisher (CP). In a casewhere the cross section is polished, the cross section is etched withnital. The cross section is then observed using an optical microscope orSEM, and a metallographic micrograph thereof is captured. If the crosssection observed with SEM is a COMPO image as shown in FIG. 1, due to adifference in a chemical composition between the coarse domain and thefine domain, a sharp contrast is made, and hence the interface betweenthe coarse domain and the fine domain can be easily discerned. Thechemical composition of the constituent phases can be measured byanalysis based on EDX or EPMA. From the result of the chemicalcomposition, the constituent phases can be simply identified. Themetallographic micrograph is binarized through, for example, imageanalysis; an area ratio of a white portion or black portion of theplated-metal-layer is measured; and in this way, area fractions of theconstituent phases can be measured. Furthermore, from the determinedarea of each coarse domain, an average equivalent circle diameter can bedetermined by calculation. Alternatively, by observing themetallographic structure of the plated-metal-layer by an Electron BackScattering Diffraction Pattern (EBSD) method, the constituent phases maybe identified, and the area fraction and the average equivalent circlediameter of the constituent phases may be determined.

In order to more specifically identify the constituent phases, themetallographic structure of the plated-metal-layer is observed as below.A thin sample is collected by cutting the plated steel sheet such thatthe cross section whose cutting direction is parallel to the thicknessdirection of the plated steel sheet is observed. The thin sample issubjected to ion milling. Alternatively, a thin sample is collected byprocessing the plated steel sheet with a Focused Ion Beam (FIB) suchthat the cross section whose cutting direction is parallel to thethickness direction of the plated steel sheet is observed. These thinsamples are observed with TEM, and the metallographic micrograph thereofis captured. The constituent phases can be accurately identified usingan electron diffraction pattern. By performing image analysis on themetallographic micrograph, the area fractions and the average equivalentcircle diameters of the constituent phases can be determined.

From XRD diffraction peaks of the plated-metal-layer, the existence ofthe constituent phases can be confirmed in the simplest way, althoughhow the constituent phases exist in a space cannot be ascertained inthis way. Here, because the diffraction peak positions of thequasicrystal phase, Mg₅₁Zn₂₀, and Mg₃₂(Zn,Al)₄₉ overlap each other, theexistence of theses phases can be confirmed, but it is difficult todistinguish them from each other.

The area fraction and the average equivalent circle diameter of theconstituent phase in the plated-metal-layer, for example, the Zn phase,the Al phase, the MgZn phase, the quasicrystal phase, or the Zn—Aleutectic may be determined through EPMA mapping image of three points ina region of 6 μm×100 μm in the aforementioned cross section of theplated-metal-layer.

The steel sheet as a base metal of the plated steel sheet according tothe embodiment is not particularly limited. As the steel sheet, it ispossible to use Al killed steel, ultra-low carbon steel, high carbonsteel, various high tensile strength steel, steel containing Ni or Cr,or the like.

Next, a method of producing a plated steel sheet according to theembodiment will be described.

The method of producing a plated steel sheet according to the embodimentincludes a hot-dip-plating process of dipping the steel sheet into ahot-dip-plating bath having an adjusted composition in order to form aplated-metal-layer on a surface of the steel sheet; a first coolingprocess of cooling the steel sheet after the hot-dip-plating processunder conditions such that an average cooling rate of theplated-metal-layer is 15° C./sec to 50° C./sec in a temperature rangewhere a temperature of the plated-metal-layer is from T_(melt)+10° C. toT_(solid-liquid), when T_(melt) in a unit of ° C. is regarded as aliquidus temperature of the plated-metal-layer and when T_(solid-liquid)in a unit of ° C. is a temperature range where the plated-metal-layer isin a coexistence state of a solid phase and a liquid phase and where avolume ratio of the solid phase to the plated-metal-layer (volume ofsolid phase/volume of plated-metal-layer) is 0.01 to 0.1; and a secondcooling process of cooling the steel sheet after the first coolingprocess under conditions such that an average cooling rate of theplated-metal-layer is 100° C./sec to 3000° C./sec in a temperature rangewhere a temperature of the plated-metal-layer is from a temperature atfinishing the first cooling process to 250° C.

A value of T_(melt) which is a liquidus temperature of theplated-metal-layer can be determined using, for example, liquidustemperatures (liquidus surface temperatures) disclosed in a non-patentdocument (Liang, P., Tarfa, T., Robinson, J. A., Wagner, S., Ochin, P.,Harmelin, M. G., Seifert, H. J., Lukas, H. L., Aldinger, F.,“Experimental Investigation and Thermodynamic Calculation of theAl—Mg—Zn system”, Thermochim. Acta, 314, 87-110 (1998)) written by Lianget al, as shown in FIG. 7. In this way, a value of T_(melt)substantially can be estimated by using the fraction of Zn, Al, and Mgcontained in the plated-metal-layer.

A value of T_(solid-liquid) can be accurately determined from an alloyphase diagram. Specifically, by using the chemical composition of theplated-metal-layer and the corresponding alloy phase diagram, a volumeratio (volume fraction) between a plurality of coexisting phases can bedetermined based on lever rule. That is, by using the alloy phasediagram, a temperature at which a volume ratio of a solid phase becomes0.01 and a temperature at which a volume ratio of the solid phasebecomes 0.1 may be determined. In the method of producing a plated steelsheet according to the embodiment, the value of T_(solid-liquid) may bedetermined using the alloy phase diagram. At this time, as the alloyphase diagram, a calculated phase diagram based on a thermodynamiccalculation system may be used. Here, because the alloy phase diagrammerely shows an equilibrium phase, the ratio between the constituentphases determined from the alloy phase diagram does not necessarilytotally agree with an actual ratio between the constituent phases in theplated-metal-layer which is being cooled. Regarding T_(solid-liquid) asa temperature range where the plated-metal-layer that is being cooled isin a coexistence state of a solid phase and a liquid phase and a volumeratio of the solid phase to the plated-metal-layer is 0.01 to 0.1, theinventors of the present invention conducted intensive investigation. Asa result, they found that T_(solid-liquid) can be empirically determinedby the following expression,{345+0.8×(T_(melt)−345)}−1≤T_(solid-liquid)<T_(melt). Therefore, in themethod of producing a plated steel sheet according to the embodiment, avalue of T_(solid-liquid) may be determined by the above expression.

In the hot-dip-plating process, the chemical composition of the platingbath is adjusted such that the chemical composition, by atomic %, of theplated-metal-layer formed on the surface of the steel sheet contains Zn:28.5% to 52%, Al: 0.5% to 10%, Ca: 0% to 3.5%, Y: 0% to 3.5%, La: 0% to3.5%, Ce: 0% to 3.5%, Si: 0% to 0.5%, Ti: 0% to 0.5%, Cr: 0% to 0.5%,Fe: 0% to 2%, Co: 0% to 0.5%, Ni: 0% to 0.5%, V: 0% to 0.5%, Nb: 0% to0.5%, Cu: 0% to 0.5%, Sn: 0% to 0.5%, Mn: 0% to 0.2%, Sr: 0% to 0.5%,Sb: 0% to 0.5%, and Pb: 0% to 0.5%, the balance of Mg and impurities.

In the embodiment, the hot-dip-plating process is selected for example.However, the method of forming the plated-metal-layer on the surface ofthe steel sheet is not limited as long as the plated-metal-layer havingthe aforementioned chemical composition can be formed on the surface ofthe steel sheet. In addition to the hot-dip-plating, spraying,sputtering, ion plating, evaporating, or electroplating may be applied.

Immediately after being pulled up out of the plating bath, theplated-metal-layer formed on the surface of the steel sheet by thehot-dip-plating process is in a molten state (liquid phase). By coolingthe plated-metal-layer in the molten state by a first cooling processand a second cooling process unique to the embodiment, theplated-metal-layer can be controlled to have the aforementionedmetallographic structure containing a quasicrystal.

In a case where a plated-metal-layer forming method other than thehot-dip-plating process is selected, by reheating the plated steelsheet, on which the plated-metal-layer is formed, by using a heatingfurnace so as to melt only the plated-metal-layer, and then cooling theplated-metal-layer by the first cooling process and the second coolingprocess unique to the embodiment, the plated-metal-layer can becontrolled to have the aforementioned metallographic structurecontaining a quasicrystal.

A melting point of the plated-metal-layer containing Mg and Zn as maincomponents is totally different from a melting point of the steel sheetas a base metal. Therefore, those skilled in the related art can easilydetermine an optimized temperature at which only the plated-metal-layeris melted and an optimized melting time.

For example, if being heated to 700° C., the plated-metal-layer iscompletely melted while the steel sheet as a base metal is not melted.Particularly, rapid heating in a high-temperature atmosphere ispreferable because the plated-metal-layer of the plated steel sheetcontacting the atmosphere is preferentially heated.

In the hot-dip-plating process, an oxygen concentration of theatmosphere at the time of dipping the steel sheet is preferably 0 ppm to100 ppm in a volume ratio; a plating tub holding the plating bath ispreferably a steel tub; T_(bath) which is a temperature of the platingbath is preferably 10° C. to 100° C. higher than T_(melt); and a time todip the steel sheet into the plating bath is preferably 1 sec to 10 sec.

When the oxygen concentration is 100 ppm or less in a volume ratio,oxidation of the plating bath can be preferably inhibited. The oxygenconcentration is more preferably 50 ppm or less in a volume ratio. Whenthe plating tub is a steel tub, an amount of inclusions in the platingbath is reduced, and hence a quasicrystal is preferably formed in themetallographic structure of the plated-metal-layer. Furthermore, in acase where the plating tube is a steel tub, wearing of the inner wallsof the plating tub can be further inhibited than in a case where theplating bath is a ceramic bath. When T_(bath) which is a temperature ofthe plating bath is 10° C. to 100° C. higher than T_(melt), theplated-metal-layer is preferably formed on the steel sheet, and a Fe—Alcontaining alloy layer is formed between the steel sheet and theplated-metal-layer. T_(bath) which is a temperature of the plating bathis more preferably 30° C. to 50° C. higher than T_(melt). When the timeto dip the steel sheet into the plating bath is 1 sec to 10 sec, theplated-metal-layer is preferably formed on the steel sheet, and a Fe—Alcontaining alloy layer is formed between the steel sheet and theplated-metal-layer. The time to dip the steel sheet into the platingbath is more preferably 2 sec to 4 sec.

In the first cooling process, it is important to control the averagecooling rate of the plated-metal-layer at the time when the temperatureof the plated-metal-layer reaches T_(solid-liquid), which is atemperature range where a volume ratio of a solid phase to the inner ofthe plated-metal-layer (liquid phase+solid phase) is 0.01 to 0.1, fromT_(melt)+10° C. as a liquidus temperature of the plated-metal-layer. Inthe first cooling process, the steel sheet on which theplated-metal-layer is formed is cooled by controlling the averagecooling rate within a range of 15° C./sec to 50° C./sec.

On the other hand, the cooling of the first cooling process may beperformed under the following conditions. In a case whereT_(melt)≥T_(solid-liquid)+10° C. is satisfied, the steel sheet after thehot-dip-plating process may be cooled under conditions such that anaverage cooling rate of the plated-metal-layer is within a range of 15°C./sec to 50° C./sec in a temperature range where the temperature of theplated-metal-layer reaches T_(solid-liquid) from T_(melt). In a casewhere T_(melt)<T_(solid-liquid)+10° C. is satisfied, the steel sheetafter the hot-dip-plating process may be cooled under conditions suchthat an average cooling rate of the plated-metal-layer is within a rangeof 15° C./sec to 50° C./sec in a temperature range where the temperatureof the plated-metal-layer reaches T_(solid-liquid) fromT_(solid-liquid)+10° C.

By the cooling performed in the first cooling process, at least one ormore kinds of the Zn phase, the quasicrystal phase, the MgZn phase, andthe Al phase are crystallized as a primary phase in theplated-metal-layer that is in a molten state (liquid phase) before thebeginning of cooling. The Zn phase, the quasicrystal phase, the MgZnphase or the Al phase that are crystallized are finally become theconstituent phase included in the coarse domain. Here, the Zn phase andthe Al phase may form a eutectic structure. It is preferable that theaverage equivalent circle diameter of a block size of the eutecticstructure is larger than 0.2 μm and becomes the coarse domain.

If the average cooling rate in the first cooling process is less than15° C./sec, a quasicrystal is not easily formed because the averagecooling rate does not reach a cooling rate of a quasicrystal phase thatis originally formed as a non-equilibrium phase. In addition, the coarsedomain which mainly includes the Zn phase, the Al phase, or the MgZnphase is less likely to be formed. In contrast, if the average coolingrate in the first cooling process is higher than 50° C./sec, aquasicrystal phase, a Zn phase, a MgZn phase, and an Al phase, or thelike, having an average equivalent circle diameter of less than 0.2 μmare formed too much. Furthermore, in some cases, the coarse domain isnot formed, and the aforementioned bimodal structure is not established.In a case where the cooling rate is extremely high, an amorphous phaseis formed too much. Therefore, the upper limit of the average coolingrate in the first cooling process is set to be 50° C./sec.

In the first cooling process, in a case where the average cooling rateof the plated-metal-layer is controlled to satisfy the aforementionedconditions from a temperature lower than T_(melt)+10° C. (in a casewhere it is controlled from a temperature lower than T_(melt) whenT_(melt)≥T_(solid-liquid)+10° C. is satisfied, or in a case where it iscontrolled from a temperature lower than T_(solid-liquid)+10° C. whenT_(melt)<T_(solid-liquid)+10° C. is satisfied), the primary phasecrystallized in the plated-metal-layer cannot become a Zn phase, aquasicrystal phase, a MgZn phase or an Al phase. Furthermore, in a casewhere the control of the average cooling rate to satisfy theaforementioned conditions is stopped at a temperature higher thanT_(solid-liquid), or in a case where the average cooling rate iscontrolled to satisfy the aforementioned conditions down to atemperature lower than T_(solid-liquid), the average equivalent circlediameter and the area fraction of the Zn phase, the Al phase, thequasicrystal phase, the MgZn phase, or the Zn—Al eutectic phase cannotbe preferably controlled. In addition, in some cases, the metallographicstructure cannot be controlled to become a bimodal structure consistingof the coarse domain and the fine domain described above. In a casewhere the cooling of the first cooling process is performed based on atemperature at which a volume ratio of a solid phase to a liquid phasein the plated-metal-layer does not become 0.01 to 0.1, the averageequivalent circle diameter and the area fraction of the Zn phase, the Alphase, or the Zn—Al eutectic phase cannot be preferably controlled.Moreover, in some cases, the metallographic structure cannot becontrolled to become a bimodal structure consisting of the coarse domainand the fine domain described above. In the first cooling process, in acase where the average cooling rate of the plated-metal-layer is lessthan 15° C./sec or more than 50° C./sec, the Zn phase, the Al phase, thequasicrystal phase, the MgZn phase, or the Zn—Al eutectic phase cannotbe the coarse domain in some cases.

In the second cooling process, it is important to control the averagecooling rate of the plated-metal-layer at the time when the temperatureof the plated-metal-layer reaches 250° C. from a temperature at a pointin time when the first cooling process ends, that is, from a temperatureat finishing the first cooling process that is within T_(solid-liquid).The steel sheet having undergone the first cooling process is cooled bycontrolling the average cooling rate to become 100° C./sec to 3,000°C./sec. The lower limit of the temperature range is preferably 200° C.,more preferably 150° C., and most preferably 100° C.

By the cooling in the second cooling process, in the plated-metal-layerin which a Zn phase, a quasicrystal phase, a MgZn phase, or an Al phaseis crystallized as a primary phase and a solid phase and a liquid phaseis in a coexistence state, at least one or more kinds of phase among aquasicrystal phase, a Mg₅₁Zn₂₀ phase, a Zn phase, an amorphous phase, ora Mg₃₂(Zn,Al)₄₉ phase that are more fine are crystallized. It ispreferable that the crystallized the quasicrystal phase, the Mg₅₁Zn₂₀phase, the Zn phase, the amorphous phase, or the Mg₃₂(Zn,Al)₄₉ phasefinally become constituent phases included in the fine domain.

In the second cooling process, in a case where the average cooling rateis controlled to satisfy the aforementioned conditions from atemperature higher or lower than T_(solid-liquid), the averageequivalent circle diameter and the area fraction of the Zn phase, the Alphase, the quasicrystal phase, the MgZn phase, or the Zn—Al eutecticphase cannot be preferably controlled. Furthermore, in some cases, themetallographic structure cannot be controlled to become a bimodalstructure consisting of the coarse domain and the fine domain describedabove. In addition, in a case where the control of the average coolingrate to satisfy the aforementioned conditions is stopped at atemperature higher than 250° C., the quasicrystal phase as anon-equilibrium phase, the Mg₅₁Zn₂₀ phase, and the Mg₃₂(Zn,Al)₄₉ phaseundergo phase decomposition in some cases. Moreover, in some cases, themetallographic structure cannot be controlled to become a bimodalstructure consisting of the coarse domain and the fine domain describedabove. In a case where the average cooling rate in the second coolingprocess is less than 100° C./sec, the quasicrystal phase, the Mg₅₁Zn₂₀phase, the Zn phase, the amorphous phase, or the Mg₃₂(Zn,Al)₄₉ phase isnot formed, or a metallographic structure containing an extremely largeamount of Mg phase is established. In addition, the quasicrystal phase,the Mg₅₁Zn₂₀ phase, the Zn phase, the amorphous phase, or theMg₃₂(Zn,Al)₄₉ phase does not becomes the fine domain in some cases. In acase where the average cooling rate in the second cooling process isgreater than 3,000° C./sec, an amorphous phase is formed too much, andhence the metallographic structure cannot be controlled to become theaforementioned bimodal structure in some cases.

As described above, T_(melt) as a liquidus temperature of theplated-metal-layer may be determined from a liquidus surface phasediagram of a ternary Zn—Al—Mg system. T_(solid-liquid) as a temperaturerange in which a volume ratio of a solid phase to the plated-metal-layerbecomes 0.01 to 0.1 may be determined from the following expression.{345+0.8×(T_(melt)−345)}−1≤T_(solid-liquid)<T_(melt). Because the amountof solid phase explosively increases around the temperature range inwhich the volume ratio of a solid phase to the plated-metal-layerbecomes 0.01 to 0.1, the cooling of the first cooling process isfinished in the temperature range. By controlling the cooling based on{345+0.8×(T_(melt)−345)}, the average equivalent circle diameter and thearea fraction of the constituent phase can be preferably controlled. Inthis way, in order to form the aforementioned plated-metal-layer, thetemperature needs to be accurately controlled.

In a method for actually measuring a temperature of theplated-metal-layer at the time of producing the plated steel sheetaccording to the embodiment, a contact-type thermocouple (K-type) may beused. By mounting the contact-type thermocouple on an original sheet, anaverage temperature of the whole plated-metal-layer can be monitored allthe time. If a pull-up rate and a thickness are mechanically controlled,and a preheating temperature of the steel sheet, a temperature of thehot-dip plating bath, or the like are standardized, it is possible tosubstantially accurately monitor a temperature of the wholeplated-metal-layer at the point in time under the production conditions.Consequently, the cooling in the first cooling process and the secondcooling process can be accurately controlled. A surface temperature ofthe plated-metal-layer may be measured using a noncontact-type radiationthermometer, although the noncontact-type thermometer is not as accurateas a contact-type.

A relationship between a surface temperature of the plated-metal-layerand an average temperature of the whole plated-metal-layer may bedetermined by cooling simulation for analyzing thermal conductivity.Specifically, based on each of the production conditions such as apreheating temperature of the steel sheet, a temperature of the hot-dipplating bath, a rate at which the steel sheet is pulled up out of theplating bath, a thickness of the steel sheet, a thickness of theplated-metal-layer, an amount of heat exchanged between theplated-metal-layer and the production facilities, and an amount of heatradiated from the plated-metal-layer, a surface temperature of theplated-metal-layer and an average temperature of the wholeplated-metal-layer may be determined, and a relationship between thesurface temperature of the plated-metal-layer and the averagetemperature of the whole plated-metal-layer may be determined. As aresult, by actually measuring the surface temperature of theplated-metal-layer at the time of producing the plated steel sheet, theaverage temperature of the whole plated-metal-layer at the point in timeunder the production conditions can be inferred. Consequently, thecooling in the first cooling process and the second cooling process canbe accurately controlled.

The cooling method in the first cooling process and the second coolingprocess is not particularly limited. As the cooling method, coolingusing a rectified high-pressure gas, mist cooling, or submersion coolingmay be performed. Here, in order to preferably control the surfacecondition of the plated-metal-layer or the formation of thequasicrystal, it is preferable to perform cooling using a rectifiedhigh-pressure gas. If H₂ or He is used, the cooling rate is increased.

As the hot-dip plating applied in the embodiment, all of the knownplating methods such as a Sendzimir method, a pre-plating method, atwo-step plating method, and a flux method can be used. As pre-plating,displacement plating, electroplating, or evaporating, or the like can beused.

In the method of producing a plated steel sheet according to theembodiment, steel used as a base metal of the plated steel sheet is notparticularly limited. The aforementioned effects are not affected by thechemical composition of steel, and Al killed steel, ultra-low carbonsteel, high carbon steel, various high tensile strength steel, steelcontaining Ni or Cr, or the like can be used.

In the method of producing a plated steel sheet according to theembodiment, each of the processes such as a steel making process, a hotrolling process, a pickling process, and a cold rolling process thatprecede the hot-dip plating process is not particularly limited. Thatis, the production conditions of the steel sheet supplied in the hot-dipplating process or the material of the steel sheet is not particularlylimited.

Here, the steel sheet supplied in the hot-dip plating process preferablyhas a temperature difference between a surface temperature and aninternal temperature. Specifically, it is preferable that a surfacetemperature of the steel sheet immediately before being dipped into theplating bath is higher than an internal temperature thereof. Forexample, a surface temperature of the steel sheet immediately beforebeing dipped into the plating bath is preferably 10° C. to 50° C. higherthan a temperature of the center of the steel sheet in the thicknessdirection thereof. In this case, immediately after being pulled up outof the plating bath, the plated-metal-layer undergoes heat extractiondue to the steel sheet, and accordingly, the plated-metal-layer can bepreferably controlled to have the aforementioned metallographicstructure containing a quasicrystal. A method for making a temperaturedifference between the surface temperature and the internal temperaturein the steel sheet immediately before being dipped into the plating bathis not particularly limited. For example, the steel sheet immediatelybefore being dipped into the plating bath may be rapidly heated in ahigh-temperature atmosphere such that only the surface temperature ofthe steel sheet is controlled to become a temperature preferable forperforming hot-dip plating. In this case, because only the surface areaof the steel sheet is heated preferentially, the steel sheet can bedipped into the plating bath in a state of having a temperaturedifference between the surface temperature and the internal temperature.

For evaluating the corrosion resistance of the plated-metal-layer, anexposure test is most preferable which makes it possible to evaluate thecorrosion resistance of the plated-metal-layer in a real environment. Byevaluating a corrosion loss of the plated-metal-layer for apredetermined period of time, whether the corrosion resistance isexcellent or poor can be evaluated.

In a case where plated-metal-layers having high corrosion resistance arecompared with each other in terms of corrosion resistance, it ispreferable to perform a long-term corrosion resistance test. Thecorrosion resistance is evaluated based on the time taken for red rustto occur. Furthermore, at the time of evaluating the corrosionresistance, it is important to consider a time period during which thesteel sheet is under protection.

In order to evaluate the corrosion resistance in a simpler way, it ispossible to use a combined cycle corrosion test or an acceleratedcorrosion test such as a salt spray test. By evaluating a corrosion lossor a period of time during which the red rust resistance lasts, whetherthe corrosion resistance is excellent or poor can be determined. In acase where plated-metal-layers having high corrosion resistance arecompared with each other in terms of corrosion resistance, it ispreferable to use a combined cycle corrosion test using ahigh-concentration aqueous NaCl solution with a concentration of around5%. If a low-concentration (1% or less) aqueous NaCl solution is used,it is difficult to determine whether the corrosion resistance isexcellent or poor.

The plated-metal-layer may also be subjected to conversion coating usingan organic or inorganic material. The plated-metal-layer according tothe embodiment contains Zn in an amount of equal to or greater than acertain level. Therefore, the plated-metal-layer according to theembodiment can be subjected to the same conversion coating as performedon a Zn group-plated steel sheet. The same will be applied to paintingperformed on a film having undergone conversion coating. Furthermore,the plated-metal-layer according to the embodiment can also be used as abase sheet of a laminated steel sheet.

It is considered that the plated steel sheet according to the embodimentcan be used particularly in places in a severely corrosive environment.The plated steel sheet according to the embodiment can be used as asubstitute for various plated steel sheets used in the fields ofbuilding materials, automobiles, consumer electronics, energy, and thelike.

EXAMPLE 1

Next, effects of an aspect of the present invention will be specificallydescribed based on examples. The conditions in the examples are merelyan example adopted for checking a possibility of embodying the presentinvention and effects thereof, and the present invention is not limitedto the example conditions. As long as the object of the presentinvention is achieved, the present invention can adopt variousconditions without departing from the gist of the present invention.

Through a hot-dip plating process, a first cooling process, and a secondcooling process under the production conditions shown in Tables 1 and 2,a plated steel sheet containing a quasicrystal was produced. A platingbath was obtained by dissolving a predetermined amount of pure metalingot. The plating bath was covered with a sealing box and then purgedwith an Ar gas such that an oxygen concentration thereof was controlledto reach a predetermined level.

As a base sheet for plating (steel sheet as a base metal of the platedsteel sheet), a hot rolled steel sheet having a thickness of 0.8 mm(carbon content: 0.2% by mass) was used. The steel sheet was cut in 100mm×200 mm. For hot-dip plating, a batch-type hot-dip plating tester wasused. During production, a temperature of a center area of the platedsteel sheet was monitored.

Before the steel sheet was dipped into the plating bath, the surface ofthe steel sheet heated to 800° C. was reduced using a N₂-5% H₂ gas in afurnace in which an oxygen concentration was controlled. The steel sheetwas air-cooled using a N₂ gas until a surface temperature of the steelsheet reached a temperature which is 20° C. higher than a temperature ofthe plating bath, and the steel sheet was dipped into the plating bathfor a predetermined time. After being dipped into the plating bath, thesteel sheet was pulled up at a pull-up rate of 100 mm/sec. When thesteel sheet was pulled up, a rectified high-pressure N₂ gas or a mixedgas of H₂ and N₂ was blown to the steel sheet from an outlet that was aparallel slit, thereby controlling an adhered amount (thickness of aplated-metal-layer) and a cooling rate.

20 (C direction: transverse direction) mm×15 (L direction: rollingdirection) mm samples were collected from 10 random sites in theprepared plated steel sheet. By dipping the samples into a 10% aqueousHCl solution for 1 second, an oxide film was removed. A metallographicstructure of a cross section (whose cutting direction was parallel to athickness direction of the plated steel sheet) of each sample wasobserved with SEM, an equivalent circle diameter or an area fraction ofeach constituent phase (each grain) was measured, and an average thereofwas calculated. The equivalent circle diameter or the area fraction ofeach constituent phase was determined through image analysis. A chemicalcomposition of each constituent phase was measured through analysisusing EPMA.

The metallographic structures of three random samples out of the tensamples were observed with an optical microscope (1,000× magnification),and a Vickers indentation was left at a target site. Base on the Vickersindentation, a 8 mm×8 mm sample was cut off. From each sample, a samplefor TEM observation was prepared by cryo-ion milling.

By analyzing an electron diffraction pattern of a main grain observedwith TEM, the constituent phase (a quasicrystal, Zn, Al, MgZn, Mg₅₁Zn₂₀,Mg₃₂(Zn,Al)₄₉, an amorphous phase, or the like) contained in themetallographic structure was identified. Furthermore, an equivalentcircle diameter or an area fraction of each constituent phase wasdetermined through image analysis, and a chemical composition of eachconstituent phase was measured through analysis using EDX as necessary.Whether or not the Mg phase exists was determined and checked by XRD. Ina case where diffraction intensity of the Mg phase in the XRDdiffraction pattern was smaller than a prescribed value, it wasdetermined that the metallographic structure of the plated-metal-layerdoes not contain the Mg phase.

The corrosion resistance, sacrificial protection, antiglare effect andappearance of the plated steel sheet of the plated-metal-layer wereevaluated. As corrosion resistance, a corrosion loss, the occurrence ofred rust, the occurrence of white rust, the occurrence of red rust in aprocessed portion, and corrosion resistance after coating wereevaluated.

The corrosion lost was evaluated by a Combined cycle Corrosion Test(CCT) based on a JASO (M609-91) cycle. Specifically, for evaluating thecorrosion loss, a 50 mm (C direction)×100 mm (L direction) sample wascut off from the prepared plated steel sheet and subjected to thecombined cycle corrosion test. The combined cycle corrosion test (CCT)was performed using a 0.5% aqueous NaCl solution, and a corrosion lossafter 150 cycles was evaluated.

In the corrosion lost evaluation, a plated steel sheet resulting in acorrosion loss of less than 20 g/m² was determined as being “Excellent”,a plated steel sheet resulting in a corrosion loss of equal to orgreater than 20 g/m² and less than 30 g/m² was determined as being“Good”, and a plated steel sheet resulting in a corrosion loss of equalto or greater than 30 g/m² was determined as being “Poor”. “Excellent”shows that the plated steel sheet is the best in the corrosion lossevaluation.

The occurrence of red rust was evaluated by the aforementioned combinedcycle corrosion test (CCT). Specifically, the produced plated steelsheet was subjected to the combined cycle corrosion test (CCT) by usinga 5% aqueous NaCl solution, and the number of test cycles in which redrust occurred in an area of greater than 5% of a planar portion of theplated steel sheet was investigated.

In the evaluation of the occurrence of red rust, a plated steel sheet inwhich the red rust was not confirmed after 300 cycles was determined asbeing “Excellent”, an plated steel sheet in which the red rust was notconfirmed after 150 cycles was determined as being “Very Good”, a platedsteel sheet in which the red rust was not confirmed after 100 cycles wasdetermined as being “Good”, and a plated steel sheet in which the redrust was confirmed before the 100th cycle was determined as being“Poor”. “Excellent” shows that the plated steel sheet is the best in theevaluation of the occurrence of red rust.

The occurrence of white rust was evaluated by a Salt Spray Test (SST)based on JIS Z2371: 2000. Specifically, the produced plated steel sheetwas subjected to the salt spray test (SST) by using a 5% aqueous NaClsolution, and a time taken for white rust to occur in an area of greaterthan 5% of a planar portion of the plated steel sheet during the testwas investigated.

In the evaluation of the occurrence of white rust, a plated steel sheetin which the white rust was not confirmed after 120 hours was determinedas being “Excellent, a plated steel sheet in which the white rust wasnot confirmed after 24 hours was determined as being “Good”, and aplated steel sheet in which the white rust was confirmed before 24 hourspassed was determined as being “Poor”. “Excellent” shows that the platedsteel sheet is the best in the evaluation of the occurrence of whiterust.

The occurrence of red rust in a processed portion was evaluated byperforming an accelerated corrosion test (CCT: Combined cycle CorrosionTest) based on JASO (M609-91) on the plated steel sheet after deepdrawing process. Specifically, the produced plated steel sheet wasprocessed in a cup shape by deep drawing process (a cylindrical drawingprocess) under the condition of a diameter of punch φ is 50 mm, ashoulder of die R is 10 mm, a shoulder of punch R is 10 mm, a draw ratiois 2.0, a blank holding pressure is 0.5 ton. The plated steel sheet wassubjected to the accelerated corrosion test, and the number of testcycles in which red rust occurred in an area of greater than 5% of theprocessed portion of the plated steel sheet was investigated.

In the evaluation of the occurrence of red rust in a processed portion,a plated steel sheet in which the red rust was not confirmed after 60cycles was determined as being “Excellent”, a plated steel sheet inwhich the red rust was not confirmed after 30 cycles was determined asbeing “Good”, and a plated steel sheet in which the red rust wasconfirmed before 30 cycles passed was determined as being “Poor”.“Excellent” shows that the plated steel sheet is the best in theevaluation of the occurrence of red rust in a processed portion.

The corrosion resistance after coating was evaluated by the test underthe following conditions. Specifically, a produced plated steel sheetwas treated with zinc phosphate-based conversion coating (Surf dyne:SDS350, manufactured by Nippon Paint Co., Ltd.), and was treated byelectrodeposition coating (PN 110 gray: manufactured by Nippon PaintCo., Ltd.). And further, a cut flaw reaching the base steel with alength of 10 mm was applied on the surface of the coated steel sheetwith a utility knife. The coated steel sheet having the cut flaw wassubjected to the combined cycle corrosion test (JASO M609-91), and wascorroded after 240 cycles. The width of a bulging of the coating layeraround the cut portion after 240 cycles was evaluated.

In the corrosion resistance after coating evaluation, a plated steelsheet resulting in that there is no blister on a flat portion and thewidth of a bulging of the coating layer from the cut flaw is within 2 mmafter 240 cycles was determined as being “Excellent”, a plated steelsheet resulting in that there is no blister and the width of a bulgingof the coating layer from the cut flaw is within 3 mm was determined asbeing “Very Good”, a plated steel sheet resulting in that there is noblister and the width of a bulging of the coating layer from the cutflaw is within 5 mm was determined as being “Good”, and a plated steelsheet resulting in that there is one or more blister or the width of abulging of the coating layer from the cut flaw is larger than 5 mm wasdetermined as being “Poor”. Here, “Excellent” shows that the platedsteel sheet is the best in the corrosion loss evaluation.

The sacrificial protection was evaluated by an electrochemicaltechnique. Specifically, The produced plated steel sheet was dipped intoa 0.5% aqueous NaCl solution, and a corrosion potential of the producedplated steel sheet was measured using a Ag/AgCl reference electrode. Inthis case, a corrosion potential of Fe is about −0.62 V.

In the evaluation of sacrificial protection, a plated steel sheet inwhich a corrosion potential was −1.0V to −0.8V with respect to theAg/AgCl reference electrode was determined as being “Excellent”, and aplated steel sheet in which a corrosion potential was not −1.0V to −0.8Vwas determined as being “Poor”. “Excellent” shows that a difference in apotential between the plated steel sheet and Fe is small, and anexcellent sacrificial protection performance is appropriatelydemonstrated.

The antiglare effect was evaluated by a spectro-colorimetric method.Usually, it is preferable to visually evaluate the antiglare effect. Inthis example, it was confirmed in advance that there is a correlationbetween the result of visual observation and an L* value obtained by acolorimeter, and then the antiglare effect was evaluated by a specularcomponent inclusion (SCI) method by using a spectral colorimeter (D65light source, 10° visual field). Specifically, an L* value of theproduced plated steel sheet was investigated by using a spectralcolorimeter CM2500d manufactured by Konica Minolta, Inc under theconditions of a measurement diameter of 8φ, 10° visual field, and a D65light source.

In the evaluation of the antiglare effect, a plated steel sheet havingan L* value of less than 75 was determined as being “Excellent”, and aplated steel sheet having an L* value of equal to or greater than 75 wasdetermined as being “Poor”. “Excellent” shows that the antiglare effectof the plated steel sheet is excellent.

The appearance of the plated steel sheet was evaluated by a test inwhich the plated steel sheet is stored in a thermohygrostat tank.Specifically, the produced plated steel sheet was stored for 72 hours ina thermohygrostat tank with a temperature of 40° C. and a humidity of95%, and an area (%) of a portion, which turned black, in a planarportion of the plated steel sheet after storage was investigated.

In the appearance evaluation, a plated steel sheet in which an area ofless than 1% of the evaluation area (45 mm×70 mm) turned black wasdetermined as being “Excellent”, a plated steel sheet in which an areaof equal to or greater than 1% and less than 3% turned black wasdetermined as being “Good”, and a plated steel sheet in which an area ofequal to or greater than 3% turned black was determined as being “Poor”.“Excellent” shows that the plated steel sheet is the best in theappearance evaluation.

The powdering property was evaluated by the amount of change in the massof the plater steel sheet before and after the cylindrical drawingprocess. Specifically, a cylindrical drawing process was applied to theproduced plated steel sheet in blank of 90φ to punch drawing of 50φ (adrawing ratio is 2.2, a low viscosity oil was applied). After thecylindrical drawing process, a tape peeling was applied to the innerportion of the plated steel sheet, and the change in the mass before andafter the test was measured. An average value obtained by the tests often times in total was used for the evaluation of the change in themass.

In the evaluation of the powdering property, a plated steel sheet inwhich a powdering amount (g/m²) is equal to or less than 1/2000 comparedto a plating deposition amount (g/m²) was determined as being“Excellent”, a plated steel sheet in which a powdering amount (g/m²) isequal to or less than 1/1000 compared to a plating deposition amount(g/m²) was determined as being “Good”, a plated steel sheet in which apowdering amount (g/m²) is more than 1/1000 compared to a platingdeposition amount (g/m²) was determined as being “Poor”. “Excellent”shows that the plated steel sheet is the best in the evaluation of thepowdering property.

The production conditions, the production results, and the evaluationresults described above are shown in the Tables 1 to 12. In the tables,an underlined numerical value is a value outside the range of thepresent invention, and a blank shows that the alloy element was notadded intentionally.

All of the plated steel sheets of Examples Nos. 1 to 23 satisfied therange of the present invention and were excellent in the corrosionresistance and the sacrificial protection. In contrast, the plated steelsheets of Comparative Example Nos. 1 to 16 do not satisfy the conditionsof the present invention and hence the corrosion resistance or thesacrificial protection thereof was insufficient.

TABLE 1 Production conditions First cooling process Temperaturedependent on chemical composition of plated-metal-layer Temper- Secondature cooling process range in Temper- Temper- Temper- which ature atureature volume actually actually actually Hot-dip-plating process ratio ofmeasured measured Average measured Average Temper- solid phase at the atthe cooling at the cooling Atmo- ature Time becomes begining end of rateof end of rate of spheric of for Liquidus 0.01 to 0.1 of cooling inplated- cooling in plated- oxygen plating dipping temper-T_(solid-liquid) cooling in first metal- second metal- concen- Materialbath steel ature Upper Lower first cooling cooling layer cooling layertration of plating T_(bath) sheet T_(melt) limit limit process process °C. / process ° C. / Classification No. ppm tub ° C. Sec ° C. ° C. ° C. °C. ° C. sec ° C. sec Comparative 1 15 Steel 400 4 350 350 348 365 349 1820 250 Example Example 1 18 Steel 420 4 350 350 348 365 349 25 20 1000Comparative 2 20 Steel 400 3 350 350 348 365 349 23 20 3000 ExampleExample 2 20 Steel 400 3 350 350 348 365 349 18 20 1000 Example 3 20Steel 400 3 355 355 352 370 353 20 20 1500 Comparative 3 20 Steel 420 3355 355 352 370 353 41 20 150 Example Example 4 25 Steel 450 3 355 355352 370 353 30 20 250 Example 5 20 Steel 420 3 375 375 368 390 369 31 201000 Example 6 20 Steel 450 4 360 360 356 375 357 40 20 150 Comparative4 15 Steel 450 3 400 400 388 415 389 21 20 1000 Example Example 7 10Steel 450 3 400 400 388 415 389 50 20 200 Example 8 20 Steel 450 4 400400 388 415 389 45 20 2000 Comparative 5 15 Steel 460 3 420 420 404 435405 43 20 150 Example Comparative 6 20 Steel 450 3 420 420 404 435 40535 20 200 Example Comparative 7 20 Steel 430 3 420 420 404 435 405 15020 90 Example Comparative 8 20 Steel 460 3 430 430 412 445 413 50 20 200Example Comparative 9 20 Steel 460 3 460 460 412 445 413 42 20 100Example Example 9 20 Steel 500 4 460 460 436 475 437 50 20 100Comparative 10 20 Steel 510 3 460 460 436 475 450 3000 20 3000 ExampleExample 10 20 Steel 500 3 450 450 428 465 429 33 20 250

TABLE 2 Production conditions First cooling process Temperaturedependent on chemical composition of plated-metal-layer Temper- Secondature cooling process range in Temper- Temper- Temper- which ature atureature volume actually actually actually ratio of measured measuredAverage measured Average Hot-dip-plating process solid phase at the atthe cooling at the cooling Atmo- Temper- Time becomes begining end ofrate of end of rate of spheric ature for Liquidus 0.01 to 0.1 of coolingin plated- cooling in plated- oxygen of plating dipping temper-T_(solid-liquid) cooling in first metal- second metal- concen- Materialbath steel ature Upper Lower first cooling cooling layer cooling layertration of plating T_(bath) sheet T_(melt) limit limit process process °C. / process ° C. / Classification No. ppm tub ° C. Sec ° C. ° C. ° C. °C. ° C. sec ° C. sec Comparative 11 20 Steel 500 3 460 460 436 475 42530 20 1000 Example Example 11 20 Steel 520 4 470 470 444 485 445 23 201000 Comparative 12 20 Steel 520 4 470 470 444 485 445 55 20 3000Example Example 12 20 Steel 550 3 510 510 476 525 477 28 20 1500 Example13 20 Steel 560 3 510 510 476 525 476 30 20 200 Example 14 20 Steel 5503 490 490 460 505 473 40 20 1000 Example 15 20 Steel 550 3 510 510 470525 477 30 20 200 Example 16 15 Steel 550 4 600 500 468 515 469 38 20100 Comparative 13 15 Steel 540 3 500 500 468 515 469 13 20 250 ExampleExample 17 10 Steel 560 3 520 520 484 536 485 15 20 3000 Example 18 20Steel 550 3 510 510 476 525 477 35 20 1500 Example 19 25 Steel 400 3 350350 348 365 349 25 20 100 Example 20 18 Steel 420 4 350 350 348 365 34915 20 3000 Example 21 20 Steel 520 4 470 470 444 485 445 17 20 2000Example 22 20 Steel 420 3 375 375 368 390 369 16 20 2500 Example 23 25Steel 400 4 350 350 348 365 349 20 20 2800 Comparative 14 25 Steel 590 3540 540 500 555 501 42 20 150 Example Comparative 15 Commerciallyavailable hot dip galvanizing Example Comparative 16 Single-phaseamorphous plated steel sheet starting to be submerged Example at 450° C.

TABLE 3 Production result Plated-metal-layer Chemical composition ofplated-metal-layer (at %) Zn At Ca Y La Ce Si Ti Cr Fe Co NiClassification No. at % at % at % at % at % at % at % at % at % at % at% at % Comparative 1 27 4 1.2 0.1 0.1 Example Example 1 28.5 1.5 0.8 0.20.2 Comparative 2 29 4 3.6 0.3 Example Example 2 30 5 1 0.2 0.1 0.2Example 3 30 2.5 1 0.2 Comparative 3 31 9 0.2 0.1 0.6 0.1 0.3 ExampleExample 4 32 4 0.2 0.2 0.5 0.1 0.2 Example 5 33 5 0.1 Example 6 34 8 0.2Comparative 4 35 4.5 3.2 0.3 0.6 Example Example 7 36 8 2.5 0.1 0.3Example 8 36 3 0.5 0.1 0.2 Comparative 5 37 0.7 0.2 Example Comparative6 38 0 1.5 0.1 Example Comparative 7 39 2 1 0.2 0.1 Example Comparative8 40 11 0.7 0.2 Example Comparative 9 40 8 5 0.2 Example Example 9 411.2 0.005 0.2 Comparative 10 41.5 3 0.3 Example Example 10 42 5 2.2 0.10.1 0.2

TABLE 4 Production result Plated-metal-layer Chemical composition ofplated-metal-layer (at %) Zn At Ca Y La Ce Si Ti Cr Fe Co NiClassification No. at % at % at % at % at % at % at % at % at % at % at% at % Comparative 11 43 1.8 1 1 1 0.2 0.3 Example Example 11 44 6 0.30.3 0.1 0.2 0.1 Comparative 12 45 9 0.2 Example Example 12 40 0.5 0.3Example 13 46 5.5 1 0.3 Example 14 46 3 1.5 0.3 Example 15 47 2.8 1 0.3Example 16 48 7 0.1 3.4 0.005 0.5 0.2 Comparative 13 49 3.5 0.8 0.2 0.30.1 Example Example 17 50 1 3 0.3 Example 18 52 10 3.5 0.3 Example 1928.5 0.5 0.2 Example 20 29 1 0.2 0.2 Example 21 43 5.8 0.3 0.3 0.1 0.20.1 Example 22 32 5.5 0.3 Example 23 28.6 0.6 0.2 Comparative 14 54 1 30.6 Example Comparative 15 Commercially available hot dip galvanizingExample Comparative 16 Single-phase amorphous plated steel sheetstarting to be submerged Example at 450° C.

TABLE 5 Production result Plated-metal-layer Chemical composition ofplated-metal-layer (at %) Value of Value of Value Ca + Y + Si + Ti + ofV Nb Cu Sn Mn Sr Sb Pb Mg La + Ce Cr Zn + Al Classification No. at % at% at % at % at % at % at % at % at % at % at % at % Comparative 1 67.61.3 0 31 Example Example 1 0.1 68.7 1 0 30 Comparative 2 63.1 3.6 0 33Example Example 2 0.1 0.1 63.3 1.2 0.1 35 Example 3 66.3 1.2 0.1 35Comparative 3 58.7 0.3 0.7 40 Example Example 4 0.1 0.2 62.5 0.9 0.1 36Example 5 0.1 0.1 61.4 0 0 38 Example 6 0.1 0.1 57.6 0 0 42 Comparative4 58.4 3.2 0 39.5 Example Example 7 0.1 53.0 2.5 0.1 44 Example 8 60.20.5 0.1 39 Comparative 5 0.6 61.5 0 0 37.7 Example Comparative 6 0.160.3 1.5 0 38 Example Comparative 7 57.7 1.2 0 41 Example Comparative 848.1 0.7 0 51 Example Comparative 9 0.1 46.7 5 0 48 Example 57.6 0 0.00542.2 Example 9 Comparative 10 55.2 0 0 44.5 Example Example 10 50.4 2.20.2 47

TABLE 6 Production result Plated-metal-layer Chemical composition ofplated-metal-layer (at %) Value of Value of Value Ca + Y + Si + Ti + ofV Nb Cu Sn Mn Sr Sb Pb Mg La + Ce Cr Zn + Al Classification No. at % at% at % at % at % at % at % at % at % at % at % at % Comparative 11 51.73 0 44.8 Example Example 11 49.0 0.6 0.1 50 Comparative 12 45.8 0 0 54Example Example 12 53.2 0 0 40.5 Example 13 0.1 0.1 47.0 1 1 51.5Example 14 0.3 0.1 0.1 48.7 1.5 0 49 Example 15 48.9 1 0 49.8 Example 1640.3 3.5 0.005 55 Comparative 13 46.1 0.8 0.2 52.5 Example Example 170.5 0.5 44.7 3 0 51 Example 18 0.005 0.3 33.9 3.5 0 82 Example 19 70.8 00 29 Example 20 69.6 0.2 0 30 Example 21 50.2 0.6 0.1 48.8 Example 220.1 0.1 61.9 0 0 37.5 Example 23 70.6 0 0 29.2 Comparative 14 41.4 3 055 Example Comparative 15 Commercially available hot dip galvanizingExample Comparative 16 Single-phase amorphous plated steel sheetstarting to be submerged Example at 450° C.

TABLE 7 Production result Plated-metal-layer Metallographic structure ofplated-metal-layer Bimodal structure Coarse domain Area fraction AreaArea Area fraction in fraction in Quasicrystal Presence fractionoutermost innermost Constituent phase Average Presence or in area ofarea of Area equivalent Value or absence plated- plated- plated-fraction Presence circle of Mg/ absence of metal- metal- metal- incoarse or diameter (Zn + of Mg bimodal layer layer layer domainClassification No. absence μm Al) phase structure % % % Type %Comparative 1 Absent — — Present Absent — — — — — Example Example 1Present 0.21 0.58 Absent Present 7 60 51 Quasicrystal/ 79/20 Zn—Aleutectic Comparative 2 Absent — — Absent Absent — — — — — ExampleExample 2 Present 0.62 0.67 Absent Present 9 45 50 Quasicrystal/ 88/11Zn—Al eutectic Example 3 Present 0.21 0.63 Absent Present 8.5 58 60Quasicrystal/ 69/30 Zn—Al eutectic Comparative 3 Absent — — AbsentAbsent — — — — — Example Example 4 Present 0.4 0.64 Absent Present 12 6870 Quasicrystal/ 55/25/19 Zn—Al eutectic/ MgZn Example 5 Present 0.60.66 Absent Present 9 48 69 Quasicrystal/ 84/15 Zn—Al eutectic Example 6Present 0.75 0.68 Absent Present 16 72 90 Quasicrystal/ 59/40 Zn—Aleutectic Comparative 4 Absent — — Absent Absent — — — — — ExampleExample 7 Present 0.95 0.70 Absent Present 15 70 91 Quasicrystal/ 66/33Zn—Al eutectic Example 8 Present 0.21 0.60 Absent Present 10 75 84Quasicrystal/ 70/25 Zn—Al eutectic Comparative 5 Absent — — AbsentAbsent — — — — — Example Comparative 6 Absent — — Absent Absent — — — —— Example Comparative 7 Absent — — Absent Absent — — — — — ExampleComparative 8 Present 1.5 0.82 Absent Present 42 58 56 Quasicrystal/Al89/10 Example Comparative 9 Absent — — Present Absent — — — — — ExampleExample 9 Present 0.25 0.60 Absent Present 7 38 90 Quasicrystal/ 80/10MgZn Comparative 10 Absent — — Absent Absent — — — — — Example Example10 Present 1 0.64 Absent Present 20 90 78 Quasicrystal/ 74/15 Zn—Aleutectic

TABLE 8 Production result Plated-metal-layer Metallographic structure ofplated-metal-layer Bimodal structure Coarse domain Area fraction AreaArea Area fraction in fraction in Quasicrystal Presence fractionoutermost innermost Constituent phase Average Presence or in area ofarea of Area equivalent or absence plated- plated- plated- fraction inPresence circle Value of absence of metal- metal- metal- coarse ordiameter Mg/ of Mg bimodal layer layer layer domain Classification No.absence μm (Zn +Al) phase structure % % % Type % Comparative 11 Present1.75 0.66 Absent Present 40 86 83 Quasicrystal/MgZn 93/3  ExampleExample 11 Present 0.95 0.69 Absent Present 26 98 99 Quasicrystal/Zn—Aleutectic 64/35 Comparative 12 Absent — — Absent Absent — — — — — ExampleExample 12 Present 0.21 0.62 Absent Present 8 80 68 Quasicrystal/Zn69/30 Example 13 Present 0.7 0.64 Absent Present 12 80 58Quasicrystal/Zn—Al eutectic 59/40 Example 14 Present 0.23 0.67 AbsentPresent 18 67 52 Quasicrystal/Zn—Al eutectic 75/24 Example 15 Present0.35 0.69 Absent Present 9 70 78 Quasicrystal/Zn 71/28 Example 16Present 0.95 0.73 Absent Present 45 89 98 Quasicrystal/ 54/45Comparative 13 Absent — — Absent Absent — — — — — Example Example 17Present 0.21 0.67 Absent Present 9 65 80 Quasicrystal/MgZn 81/4  Example18 Present 0.95 0.82 Absent Present 34 98 99 Zn—Al eutectic/Quasicrystal52/47 Example 19 Present 0.21 0.63 Present Present 8 53 49Quasicrystal/MgZn/Mg 80/18/1 Example 20 Present 0.16 0.67 Absent Present5 45 43 MgZn/Zn 65/25 Example 21 Present 0.18 0.63 Absent Present 12 5859 Zn—Al eutectic/Quasicrystal 56/43 Example 22 Present 0.17 0.56 AbsentPresent 9 45 59 ZnAl eutectic 72 Example 23 Present 0.16 0.67 PresentPresent 7 49 47 MgZn/Mg 38/1  Comparative 14 Absent — — Absent Absent —— — — — Example Comparative 15 Commercially available hot dipgalvanizing Example Comparative 16 Single-phase amorphous plated steelsheet starting to be submerged Example at 450° C.

TABLE 9 Production result Plated-metal-layer Metallographic structure ofplated-metal-layer Bimodal structure Fine domain Area fraction Fe—Alcontaining alloy layer Area Constituent phase Thickness ConstituentThickness Area fraction in Area D of phase of Fe—Al fraction in centerarea fraction plated- contained containing plated- of plated- in finemetal- Presence in Fe—Al alloy metal- metal- domain layer of containinglayer Classification No. layer % layer % Type % μm absence alloy layernm Comparative 1 — — — — 18 Present Fe₆Al₂/Al₃₂Fe 30 Example Example 193 93 Mg₅₁Zn₂₀ 99 15 Present Fe₆Al₂/Al₃₂Fe 30 Comparative 2 — — — — 15Present Fe₆Al₂/Al₃₂Fe 30 Example Example 2 91 95 Mg₅₁Zn₂₀ 97 18 PresentFe₆Al₂/Al₃₂Fe 30 Example 3 91.5 97 Mg₅₁Zn₂₀ 89 13 Present Fe₆Al₂/Al₃₂Fe30 Comparative 3 — — — — 15 Present Fe₆Al₂/Al₃₂Fe 30 Example Example 488 94 Mg₅₁Zn₂₀ 99 16 Present Fe₆Al₂/Al₃₂Fe 30 Example 5 91 97Mg₅₁Zn₂₀/Zn 94/5 10 Present Fe₆Al₂/Al₃₂Fe 20 Example 6 84 91 Mg₅₁Zn₂₀ 9913 Present Fe₆Al₂/Al₃₂Fe 30 Comparative 4 — — — — 15 PresentFe₆Al₂/Al₃₂Fe 30 Example Example 7 85 92 Mg₅₁Zn₂₀ 99 15 PresentFe₆Al₂/Al₃₂Fe 30 Example 8 90 98 Mg₅₁Zn₂₀ 99 12 Present Fe₆Al₂/Al₃₂Fe 20Comparative 5 — — — — 15 Absent — — Example Comparative 6 — — — — 15Absent — — Example Comparative 7 — — — — 13 Present Fe₆Al₂/Al₃₂Fe 30Example Comparative 8 58 60 Mg₅₁Zn₂₀/ 90/9 14 Present Fe₆Al₂/Al₃₂Fe 20Example Mg32(ZnAl)₄₉ Comparative 9 — — — — 15 Present Fe₆Al₂/Al₃₂Fe 20Example Example 9 93 99 Mg₅₁Zn₂₀ 99 16 Present Fe₆Al₂/Al₃₂Fe 20Comparative 10 — — — — 14 Present Fe₆Al₂/Al₃₂Fe 20 Example Example 10 8087 Mg₅₁Zn₂₀ 99 11 Present Fe₆Al₂/Al₃₂Fe 20

TABLE 10 Production result Plated-metal-layer Metallographic structureof plated-metal-layer Bimodal structure Fine domain Area fraction Fe—Alcontaining alloy layer Area Constituent phase Thickness ConstituentThickness Area fraction in Area D of phase of Fe—Al fraction in centerarea fraction plated- contained containing plated- of plated- in finemetal- Presence in Fe—Al alloy metal- metal- domain layer of containinglayer Classification No. layer % layer % Type % μm absence alloy layernm Comparative 11 60 65 Mg₅₁Zn₂₀ 99 13 Present Fe₆Al₂/Al₃₂Fe 20 ExampleExample 11 74 82 Mg₅₁Zn₂₀/ 95/6  16 Present Fe₆Al₂/Al₃₂Fe 20Mg32(ZnAl)₄₉ Comparative 12 — — — — 12 Present Fe₆Al₂/Al₃₂Fe 20 ExampleExample 12 92 98 Mg₅₁Zn₂₀/Zn 84/15 13 Absent — — Example 13 88 93Mg₅₁Zn₂₀/Zn 86/12 15 Present Fe₆Al₂/Al₃₂Fe 20 Example 14 82 87 Mg₅₁Zn₂₀94 13 Present Fe₆Al₂/Al₃₂Fe 30 Example 15 91 98 Mg₅₁Zn₂₀/Zn 80/19 12Present Fe₆Al₂/Al₃₂Fe 20 Example 16 55 60 Mg₅₁Zn₂₀/Zn 86/13 16 PresentFe₆Al₂/Al₃₂Fe 20 Comparative 13 — — — — 15 Present Fe₆Al₂/Al₃₂Fe 30Example Example 17 91 98 Mg51Zn20/Zn/ 92/16/1 12 Absent — — AmorphousExample 18 66 73 Mg₅₁Zn₂₀ 99 18 Present Fe₆Al₂/Al₃₂Fe 40 Example 19 9297 Mg₅₁Zn₂₀ 99 18 Absent — — Example 20 95 99 Mg51Zn20/ 94/4/1 12 Absent— — Quasicrystal/ Amorphous Example 21 88 93 Mg₅₁Zn₂₀/ 94/5  16 PresentFe₆Al₂/Al₃₂Fe 20 Mg32(ZnAl)₄₉ Example 22 91 96 Mg51Zn20/ 92/4/3 10Present Fe₆Al₂/Al₃₂Fe 20 Quasicrystal Example 23 93 98 Mg51Zn20/ 93/3/118 Absent — — Quasicrystal/ Amorphous Comparative 14 — — — — 15 Absent —— Example Comparative 15 Commercially available hot dip galvanizingExample Comparative 16 Single-phase amorphous plated steel sheetstarting to be submerged Example at 450° C.

TABLE 11 Evaluation result Evaluation of corrosion resistance EvaluationEvaluation of red rust Evaluation of Evaluation Processed Evaluation ofcorrosion occurrence portion Corrosion of appearance resistanceCorrosion Evaluation of white (Deep loss after sacrificial Antiglare(storage after Classification No. Loss of red rust rust drawing) coatingprotection effect test powdering Comparative 1 Poor Poor Poor Poor PoorPoor Poor Poor Poor Example Example 1 Good Excellent Good ExcellentExcellent Good Excellent Good Excellent Comparative 2 Poor Poor PoorPoor Poor Poor Poor Good Poor Example Example 2 Good Excellent ExcellentExcellent Excellent Good Excellent Good Good Example 3 Good ExcellentGood Excellent Excellent Good Poor Good Excellent Comparative 3 PoorPoor Poor Poor Poor Poor Poor Good Poor Example Example 4 Good ExcellentExcellent Excellent Excellent Good Excellent Good Excellent Example 5Excellent Excellent Excellent Good Good Good Poor Excellent ExcellentExample 6 Excellent Good Good Good Good Good Excellent Excellent GoodComparative 4 Poor Poor Poor Poor Poor Poor Poor Excellent Poor ExampleExample 7 Excellent Excellent Excellent Excellent Very Good GoodExcellent Excellent Good Example 8 Excellent Excellent ExcellentExcellent Excellent Good Poor Excellent Excellent Comparative 5 PoorPoor Poor Poor Poor Poor Excellent Excellent Poor Example Comparative 6Poor Excellent Poor Poor Poor Poor Poor Excellent Poor ExampleComparative 7 Poor Poor Poor Poor Poor Poor Poor Poor Poor ExampleComparative 8 Excellent Excellent Excellent Poor Poor Excellent PoorExcellent Good Example Comparative 9 Poor Poor Poor Poor Poor Poor PoorExcellent Poor Example Example 9 Excellent Good Excellent ExcellentExcellent Good Excellent Poor Excellent Comparative 10 Poor Poor PoorPoor Poor Poor Poor Excellent Excellent Example Example 10 ExcellentExcellent Excellent Good Good Very Good Poor Excellent Good

TABLE 12 Evaluation result Evaluation of corrosion resistance EvaluationEvaluation of red rust Evaluation of Evaluation Processed Evaluation ofcorrosion occurrence portion Corrosion of appearance resistanceCorrosion Evaluation of white (Deep loss after sacrificial Antiglare(storage after Classification No. Loss of red rust rust drawing) coatingprotection effect test powdering Comparative 11 Excellent ExcellentExcellent Poor Poor Excellent Poor Excellent Excellent Example Example11 Excellent Excellent Excellent Good Good Excellent Poor ExcellentExcellent Comparative 12 Poor Poor Poor Poor Poor Poor Poor ExcellentPoor Example Example 12 Excellent Good Good Excellent Excellent GoodPoor Excellent Excellent Example 13 Excellent Excellent Good ExcellentVery Good Good Excellent Excellent Good Example 14 Excellent ExcellentGood Excellent Excellent Good Excellent Excellent Excellent Example 15Excellent Excellent Good Excellent Excellent Good Poor ExcellentExcellent Example 16 Excellent Excellent Good Good Good ExcellentExcellent Excellent Good Comparative 13 Poor Poor Poor Poor Poor PoorPoor Excellent Poor Example Example 17 Excellent Excellent ExcellentExcellent Excellent Good Poor Excellent Excellent Example 18 ExcellentExcellent Excellent Good Good Excellent Excellent Excellent Good Example19 Good Good Good Good Good Good Poor Poor Excellent Example 20 GoodGood Good Good Good Good Poor Poor Excellent Example 21 ExcellentExcellent Excellent Good Good Excellent Poor Excellent Good Example 22Excellent Excellent Excellent Good Good Good Poor Excellent ExcellentExample 23 Good Good Good Good Good Good Poor Poor Excellent Comparative14 Poor Poor Poor Poor Poor Poor Poor Excellent Poor Example Comparative15 Poor Poor Poor Poor Poor Poor Poor Excellent Excellent ExampleComparative 16 Poor Poor Poor Poor Poor Poor Poor Poor Poor Example

INDUSTRIAL APPLICABILITY

According to the above aspects of the present invention, it is possibleto provide the plated steel sheet which is further excellent in thecorrosion resistance requested for applying building materials,automobiles, consumer electronics or the like. Therefore, it is possibleto prolong the useful life of the materials as compared with theconventional surface-treated steel sheets. Accordingly, the presentinvention has significant industrial applicability.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

1: STEEL SHEET

2: PLATED-METAL-LAYER

2 a: COARSE DOMAIN

2 b: FINE DOMAIN

2 a 1, 2 a 2, 2 b 1, 2 b 2: LOCAL AREA

The invention claimed is:
 1. A plated steel sheet with a quasicrystal,comprising a steel sheet and a plated-metal-layer arranged on a surfaceof the steel sheet, wherein: the plated-metal-layer comprises: as achemical composition, by atomic %: 28.5% to 52% of Zn, 0.5% to 10% ofAl, 0% to 3.5% of Ca, 0% to 3.5% of Y, 0% to 3.5% of La, 0% to 3.5% ofCe, 0% to 0.5% of Si, 0% to 0.5% of Ti, 0% to 0.5% of Cr, 0% to 2% ofFe, 0% to 0.5% of Co, 0% to 0.5% of Ni, 0% to 0.5% of V, 0% to 0.5% ofNb, 0% to 0.5% of Cu, 0% to 0.5% of Sn, 0% to 0.2% of Mn, 0% to 0.5% ofSr, 0% to 0.5% of Sb, 0% to 0.5% of Pb, and a balance of Mg andimpurities; and as a metallographic structure, a quasicrystal phasewhich is defined as a phase in which a magnesium content, a zinccontent, and an aluminum content expressed in atomic % in thequasicrystal phase satisfy 0.5≤Mg/(Zn+Al)≤0.83; and an averageequivalent circle diameter of the quasicrystal phase is equal to orlarger than 0.01 μm and equal to or smaller than 1 μm.
 2. The platedsteel sheet with a quasicrystal according to claim 1, wherein a calciumcontent, an yttrium content, a lanthanum content, and a cerium contentexpressed in atomic % in the chemical composition of theplated-metal-layer satisfy 0.3%≤Ca+Y+La+Ce≤3.5%.
 3. The plated steelsheet with a quasicrystal according to claim 1, wherein a siliconcontent, a titanium content, and a chromium content expressed in atomic% in the chemical composition of the plated-metal-layer satisfy0.005%≤Si+Ti+Cr≤0.5%.
 4. The plated steel sheet with a quasicrystalaccording to claim 1, wherein a zinc content and an aluminum contentexpressed in atomic % in the chemical composition of theplated-metal-layer satisfy30%≤Zn+Al≤52%.
 5. The plated steel sheet with a quasicrystal accordingto claim 1, wherein: when viewed in a cross section, whose cuttingdirection is parallel to a thickness direction of theplated-metal-layer, the metallographic structure of theplated-metal-layer is a bimodal structure which comprises a fine domain,which is a domain comprising a grain having an equivalent circlediameter of 0.2 μm or smaller, and a coarse domain, which is a domaincomprising a grain having an equivalent circle diameter of larger than0.2 μm; the coarse domain comprises at least one selected from thequasicrystal phase, a Zn phase, an Al phase and a MgZn phase; the finedomain comprises at least one selected from a Mg₅₁Zn₂₀ phase, a Znphase, an amorphous phase, a Mg₃₂(Zn,Al)₄₉ phase; and the averageequivalent circle diameter of the quasicrystal phase is larger than 0.2μm and equal to or smaller than 1 μm.
 6. The plated steel sheet with aquasicrystal according to claim 1, wherein: when viewed in a crosssection, whose cutting direction is parallel to a thickness direction ofthe plated-metal-layer, the metallographic structure of theplated-metal-layer is a bimodal structure which comprises a fine domain,which is a domain comprising a grain having an equivalent circlediameter of 0.2 μm or smaller, and a coarse domain, which is a domaincomprising a grain having an equivalent circle diameter of larger than0.2 μm; the coarse domain comprises at least one selected from a Znphase, an Al phase and a MgZn phase; the fine domain comprises at leastone selected from the quasicrystal phase, a Mg₅₁Zn₂₀ phase, a Zn phase,an amorphous phase, a Mg₃₂(Zn,Al)₄₉ phase; and the average equivalentcircle diameter of the quasicrystal phase is equal to or larger than0.01 μm and equal to or smaller than 0.2 μm.
 7. The plated steel sheetwith a quasicrystal according to claim 5, wherein an area fraction ofthe coarse domain in the metallographic structure is equal to or morethan 5% and equal to or less than 50%, and an area fraction of the finedomain in the metallographic structure is equal to or more than 50% andequal to or less than 95%.
 8. The plated steel sheet with a quasicrystalaccording to claim 6, wherein an area fraction of the coarse domain inthe metallographic structure is equal to or more than 5% and equal to orless than 50%, and an area fraction of the fine domain in themetallographic structure is equal to or more than 50% and equal to orless than 95%.
 9. The plated steel sheet with a quasicrystal accordingto claim 5, wherein an area fraction of the quasicrystal phase includedin the coarse domain is equal to or more than 80% and less than 100% inthe coarse domain, and an area fraction in total of the Mg₅₁Zn₂₀ phase,the Zn phase, the amorphous phase, and the Mg₃₂(Zn,Al)₄₉ phase includedin the fine domain is equal to or more than 80% and less than 100% inthe fine domain.
 10. The plated steel sheet with a quasicrystalaccording to claim 6, wherein an area fraction in total of the Zn phase,the Al phase, and the MgZn phase included in the coarse domain is equalto or more than 80% and less than 100% in the coarse domain, and an areafraction of the quasicrystal phase included in the fine domain is morethan 0% and less than 10% in the fine domain.
 11. The plated steel sheetwith a quasicrystal according to claim 5, wherein, when viewed in thecross section and when a thickness of the plated-metal-layer is regardedas D, an area from a surface of the plated-metal-layer toward the steelsheet in the thickness direction to 0.05×D is regarded as an outermostarea of the plated-metal-layer, and an area from an interface betweenthe steel sheet and the plated-metal-layer toward the plated-metal-layerin the thickness direction to 0.05×D is regarded as an innermost area ofthe plated-metal-layer, an area fraction of the coarse domain in theoutermost area of the plated-metal-layer is equal to or more than 7% andless than 100% and an area fraction of the coarse domain in theinnermost area of the plated-metal-layer is equal to or more than 7% andless than 100%, and when an area except for the outermost area and theinnermost area of the plated-metal-layer is regarded as a main-body areaof the plated-metal-layer, an area fraction of the fine domain in themain-body area of the plated-metal-layer is equal to or more than 50%and less than 100%.
 12. The plated steel sheet with a quasicrystalaccording to claim 6, wherein, when viewed in the cross section and whena thickness of the plated-metal-layer is regarded as D, an area from asurface of the plated-metal-layer toward the steel sheet in thethickness direction to 0.05×D is regarded as an outermost area of theplated-metal-layer, and an area from an interface between the steelsheet and the plated-metal-layer toward the plated-metal-layer in thethickness direction to 0.05×D is regarded as an innermost area of theplated-metal-layer, an area fraction of the coarse domain in theoutermost area of the plated-metal-layer is equal to or more than 7% andless than 100% and an area fraction of the coarse domain in theinnermost area of the plated-metal-layer is equal to or more than 7% andless than 100%, and when an area except for the outermost area and theinnermost area of the plated-metal-layer is regarded as a main-body areaof the plated-metal-layer, an area fraction of the fine domain in themain-body area of the plated-metal-layer is equal to or more than 50%and less than 100%.
 13. The plated steel sheet with a quasicrystalaccording to claim 1, wherein a Mg phase is absent in the metallographicstructure of the plated-metal-layer.
 14. The plated steel sheet with aquasicrystal according to claim 1, further comprising a Fe—Al containingalloy layer, wherein the Fe—Al containing alloy layer is arrangedbetween the steel sheet and the plated-metal-layer, the Fe—Al containingalloy layer comprises at least one selected from Fe₅Al₂ and Al_(3.2)Fe,and a thickness of the Fe—Al containing alloy layer is equal to or morethan 10 nm and equal to or less than 1000 nm.
 15. A method of producingthe plated steel sheet with a quasicrystal according to claim 1,comprising: a hot-dip-plating process comprising dipping a steel sheetinto a hot-dip-plating bath having an adjusted composition in order toform a plated-metal-layer on a surface of the steel sheet; a firstcooling process comprising cooling the steel sheet after thehot-dip-plating process such that an average cooling rate of theplated-metal-layer is equal to or faster than 15° C./sec and equal to orslower than 50° C./sec in a temperature range where a temperature of theplated-metal-layer is from T_(melt)+10° C. to T_(solid-liquid), when theT_(melt) in unit of ° C. is regarded as a liquidus temperature of theplated-metal-layer and when the T_(solid-liquid) in unit of ° C. is atemperature range where the plated-metal-layer is in a coexistence stateof a solid phase and a liquid phase and where a volume ratio of thesolid phase to the plated-metal-layer is equal to or more than 0.01 andequal to or less than 0.1; and a second cooling process comprisingcooling the steel sheet after the first cooling process such that anaverage cooling rate of the plated-metal-layer is equal to or fasterthan 100° C./sec and equal to or slower than 3000° C./sec in atemperature range where a temperature of the plated-metal-layer is froma temperature at finishing the first cooling process to 250° C.
 16. Amethod of producing the plated steel sheet with a quasicrystal accordingto claim 15, in the hot-dip-plating process: wherein an oxygenconcentration of an atmosphere at dipping the steel sheet is 100 ppm orless in volume ratio; a plating tub to hold the hot-dip-plating bath isa steel tub; T_(bath) which is a temperature of the hot-dip-plating bathis equal to or higher than 10° C. and equal to or lower than 100° C.higher than the T_(melt); and a time for dipping the steel sheet intothe hot-dip-plating bath is equal to or longer than 1 sec and equal toor shorter than 10 sec.
 17. A method of producing the plated steel-sheetwith a quasicrystal according to claim 2, comprising: a hot-dip-platingprocess comprising dipping a steel sheet into a hot-dip-plating bathhaving an adjusted composition in order to form a plated-metal-layer ona surface of the steel sheet; a first cooling process comprising coolingthe steel sheet after the hot-dip-plating process such that an averagecooling rate of the plated-metal-layer is equal to or faster than 15°C./sec and equal to or slower than 50° C./sec in a temperature rangewhere a temperature of the plated-metal-layer is from T_(melt)+10° C. toT_(solid-liquid), when the T_(melt) in unit of ° C. is regarded as aliquidus temperature of the plated-metal-layer and when theT_(solid-liquid) in unit of ° C. is a temperature range where theplated-metal-layer is in a coexistence state of a solid phase and aliquid phase and where a volume ratio of the solid phase to theplated-metal-layer is equal to or more than 0.01 and equal to or lessthan 0.1; and a second cooling process comprising cooling the steelsheet after the first cooling process such that an average cooling rateof the plated-metal-layer is equal to or faster than 100° C./sec andequal to or slower than 3000° C./sec in a temperature range where atemperature of the plated-metal-layer is from a temperature at finishingthe first cooling process to 250° C.
 18. A method of producing theplated steel sheet with a quasicrystal according to claim 3, comprising:a hot-dip-plating process comprising dipping a steel sheet into ahot-dip-plating bath having an adjusted composition in order to form aplated-metal-layer on a surface of the steel sheet; a first coolingprocess comprising cooling the steel sheet after the hot-dip-platingprocess such that an average cooling rate of the plated-metal-layer isequal to or faster than 15° C./sec and equal to or slower than 50°C./sec in a temperature range where a temperature of theplated-metal-layer is from T_(melt)+10° C. to T_(solid-liquid), when theT_(melt) in unit of ° C. is regarded as a liquidus temperature of theplated-metal-layer and when the T_(solid-liquid) in unit of ° C. is atemperature range where the plated-metal-layer is in a coexistence stateof a solid phase and a liquid phase and where a volume ratio of thesolid phase to the plated-metal-layer is equal to or more than 0.01 andequal to or less than 0.1; and a second cooling process comprisingcooling the steel sheet after the first cooling process such that anaverage cooling rate of the plated-metal-layer is equal to or fasterthan 100° C./sec and equal to or slower than 3000° C./sec in atemperature range where a temperature of the plated-metal-layer is froma temperature at finishing the first cooling process to 250° C.
 19. Amethod of producing the plated steel sheet with a quasicrystal accordingto claim 4, comprising: a hot-dip-plating process comprising dipping asteel sheet into a hot-dip-plating bath having an adjusted compositionin order to form a plated-metal-layer on a surface of the steel sheet; afirst cooling process comprising cooling the steel sheet after thehot-dip-plating process such that an average cooling rate of theplated-metal-layer is equal to or faster than 15° C./sec and equal to orslower than 50° C./sec in a temperature range where a temperature of theplated-metal-layer is from T_(melt)+10° C. to T_(solid-liquid), when theT_(melt) in unit of ° C. is regarded as a liquidus temperature of theplated-metal-layer and when the T_(solid-liquid) in unit of ° C. is atemperature range where the plated-metal-layer is in a coexistence stateof a solid phase and a liquid phase and where a volume ratio of thesolid phase to the plated-metal-layer is equal to or more than 0.01 andequal to or less than 0.1; and a second cooling process comprisingcooling the steel sheet after the first cooling process such that anaverage cooling rate of the plated-metal-layer is equal to or fasterthan 100° C./sec and equal to or slower than 3000° C./sec in atemperature range where a temperature of the plated-metal-layer is froma temperature at finishing the first cooling process to 250° C.
 20. Amethod of producing the plated steel sheet with a quasicrystal accordingto claim 5, comprising: a hot-dip-plating process comprising dipping asteel sheet into a hot-dip-plating bath having an adjusted compositionin order to form a plated-metal-layer on a surface of the steel sheet; afirst cooling process comprising cooling the steel sheet after thehot-dip-plating process such that an average cooling rate of theplated-metal-layer is equal to or faster than 15° C./sec and equal to orslower than 50° C./sec in a temperature range where a temperature of theplated-metal-layer is from T_(melt)+10° C. to T_(solid-liquid), when theT_(melt) in unit of ° C. is regarded as a liquidus temperature of theplated-metal-layer and when the T_(solid-liquid) in unit of ° C. is atemperature range where the plated-metal-layer is in a coexistence stateof a solid phase and a liquid phase and where a volume ratio of thesolid phase to the plated-metal-layer is equal to or more than 0.01 andequal to or less than 0.1; and a second cooling process comprisingcooling the steel sheet after the first cooling process such that anaverage cooling rate of the plated-metal-layer is equal to or fasterthan 100° C./see and equal to or slower than 3000° C./sec in atemperature range where a temperature of the plated-metal-layer is froma temperature at finishing the cooling process to 250° C.